1. Fix the "flash message" layout problem for different cases
* I am sure most of the users should have ever seen the ugly
center-aligned error message with multiple lines.
2. Fix inconsistent "Details" flash message EOL handling, sometimes
`\n`, sometimes `<br>`
* Now, always use "\n" and use `<pre>` to render
3. Remove SanitizeHTML template func because it is not useful and can be
easily abused.
* But it is still kept for mail templates, for example:
https://github.com/go-gitea/gitea/issues/36049
4. Clarify PostProcessCommitMessage's behavior and add FIXME comment
By the way: cleaned up some devtest pages, move embedded style block to
CSS file
1. correct the modal usage on "admin email list" page (then
`web_src/js/features/admin/emails.ts` is removed)
2. use `addDelegatedEventListener` instead of `jQuery().on`
3. more jQuery related changes and remove jQuery from
`web_src/js/features/common-button.ts`
4. improve `confirmModal` to make it support header, and remove
incorrect double-escaping
5. fix more typescript related types
6. fine tune devtest pages and add more tests
