Allow migrating attachments from other places

[JohnNiang]

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:

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 #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?

None

[guqing]

/approve

我配置

halo:
  attachment:
    resource-mappings:
      - pathPattern: /route-prefix/**
        locations:
          - test

然后在 {halo-work-dir}/attachments/test/ 放入一张 me.jpeg
最终访问 http://localhost:8090/route-prefix/me.jpeg 成功，访问没有的图片 404 符合期望

[f2c-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: guqing

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[ruibaby]

/lgtm