Fix the problem of not initializing default theme correctly (halo-dev…

…#3194)

#### What type of PR is this?

/kind bug
/area core
/milestone 2.2.x

#### What this PR does / why we need it:

Make requires field of Theme not required.

#### Which issue(s) this PR fixes:

Fixes halo-dev#3177

#### Special notes for your reviewer:

1. Clear Halo working directory
2. Start Halo freshly

#### Does this PR introduce a user-facing change?

```release-note
None
```

chore: bump theme-earth to 1.1.1 (halo-dev#3082)

#### What type of PR is this?

/kind improvement

#### What this PR does / why we need it:

将 theme-earth 默认主题升级到 1.1.1

#### Does this PR introduce a user-facing change?


```release-note
None
```

[release-2.0] Avoid NPE problems when querying the list of posts in t…

…he console (halo-dev#3075)

This is an automated cherry-pick of halo-dev#3068

/assign ruibaby

```release-note
None
```

Refine exception detail with i18n (halo-dev#3042)

#### What type of PR is this?

/kind feature
/kind api-change
/area core
/milestone 2.1.x

#### What this PR does / why we need it:

- Configuring message source location and name enables i18n message resolution.
- Simple global error handler.
- Refactor some exceptions with `ResponseStatusException` to control what HTTP status and problem detail need to be returned.

**TODO**

- [x] Add more UTs.
#### Which issue(s) this PR fixes:

Fixes halo-dev#3020

#### Special notes for your reviewer:

Steps to test:

1. Try to refine `src/main/resources/config/i18n/messages_zh.properties` and switch Browser language with Chinese.
2. Delibrately make a mistake as you wish and see the error tips in console.
3. Try to access one page which doesn't exist and see the rendered result.

#### Does this PR introduce a user-facing change?

```release-note
完成系统异常的国际化
```

Bump version to 2.0.2-SNAPSHOT for next release (halo-dev#2977)

#### What type of PR is this?

/kind cleanup
/area core

#### What this PR does / why we need it:

Bump version to 2.0.2-SNAPSHOT for next release.

#### Does this PR introduce a user-facing change?

```release-note
None
```

[release-2.0] Set docker image name properly when releasing (halo-dev…

…#2873)

This is an automated cherry-pick of halo-dev#2872

/assign JohnNiang

```release-note
None
```

Allow migrating attachments from other places (halo-dev#2807)

#### What type of PR is this?

/kind improvement
/area core
/milestone 2.0.0

#### What this PR does / why we need it:

This PR provides an ability to migrate attachments from other places, like Halo 1.x or Wordpress.

We could simply configure resource mappings to support attachments migration:

```yaml
halo:
  attachment:
    resource-mappings:
      - pathPattern: /upload/**
        locations:
          - upload
          - migrate-from-1.x
      - pathPattern: /wp-content/uploads/**
        locations:
          - migrate-from-wp
```

Meanwhile, I refactored LocalAttachmentUploadHandler for managing attachments from migration in the future.

#### Which issue(s) this PR fixes:

Fixes halo-dev#2585

#### Special notes for your reviewer:

**Steps to test:**
1. Try to configure the resource mappings
2. Put some static resources into the corresponding location
3. Access it from Browser

At last, please make sure the functionalities of attachment are ok as before.

#### Does this PR introduce a user-facing change?

```release-note
None
```

Fix the problem that lucene crashes when searching for posts (halo-de…

…v#2791)

#### What type of PR is this?

/kind bug
/area core
/milestone 2.0.0-rc.2

#### What this PR does / why we need it:

This PR mainly refactor PostEventListener into PostEventReconciler to reconcile post events synchronously. After that, the concurrent issue will be gone.

#### Which issue(s) this PR fixes:

Fixes halo-dev#2756
Fixes halo-dev#2757

#### Does this PR introduce a user-facing change?

```release-note
None
```

fix: cannot be published successfully after post or single page unpub…

…lished (halo-dev#2762)

#### What type of PR is this?
/kind bug
/area core
/milestone 2.0.0-rc.1

#### What this PR does / why we need it:
修复文章和自定义页面取消发布后无法再继续发布的问题

#### Special notes for yourd reviewer:
how to test it?
1. create a post and publsih it
2. unpublish it then republish
3. excepted to b published successfully

/cc @halo-dev/sig-halo 
#### Does this PR introduce a user-facing change?
```release-note
None
```

chore: release 1.6.1 (halo-dev#2748)

#### What type of PR is this?

/kind improvement

#### What this PR does / why we need it:

发布 1.6.1 版本。

```release-note
None
```