Allow inserting HTML fragments in email templates

[tcitworld]

This is a proposal, this PR is here for discussions.

Email templates currently only allow to add text bodies, lists and buttons. It's not possible to add formatting or other elements. Also, text bodies are always centered, which can be hard to read when there a lot of text. This provides a way to inject arbitrary HTML in emails, which can be useful in some cases.

One of these cases I have is nextcloud/announcementcenter#644, which would be achieved otherwise by creating an IMessage from scratch, but that prevents consistency and theming.

Right now the HTML fragment is really just injected in the HTML body, but we could validate (very basically, at a XML level) and sanitize it (using SF's HTML Sanitizer).

Another option is to allow for Markdown fragments directly, and do the rendering here (brings another specific dependency though).

TODO

• Add tests to tests/lib/Mail/EMailTemplateTest.php
• Add docs for the new feature

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[come-nc]

I don’t have anything against it, but it does feel like there should be some basic safety check.