Partially for #24457
Major changes:
1. The old `signedUserNameStringPointerKey` is quite hacky, use
`ctx.Data[SignedUser]` instead
2. Move duplicate code from `Contexter` to `CommonTemplateContextData`
3. Remove incorrect copying&pasting code `ctx.Data["Err_Password"] =
true` in API handlers
4. Use one unique `RenderPanicErrorPage` for panic error page rendering
5. Move `stripSlashesMiddleware` to be the first middleware
6. Install global panic recovery handler, it works for both `install`
and `web`
7. Make `500.tmpl` only depend minimal template functions/variables,
avoid triggering new panics
Screenshot:
<details>
</details>
The old `HTMLRender` is not ideal.
1. It shouldn't be initialized multiple times, it consumes a lot of
memory and is slow.
2. It shouldn't depend on short-lived requests, the `WatchLocalChanges`
needs a long-running context.
3. It doesn't make sense to use FuncsMap slice.
HTMLRender was designed to only work for GItea's specialized 400+
templates, so it's good to make it a global shared instance.
Remove `[repository.editor] PREVIEWABLE_FILE_MODES` setting that seemed
like it was intended to support this but did not work. Instead, whenever
viewing a file shows a preview, also have a Preview tab in the file
editor.
Add new `/markup` web and API endpoints with `comment`, `gfm`,
`markdown` and new `file` mode that uses a file path to determine the
renderer.
Remove `/markdown` web endpoint but keep the API for backwards and
GitHub compatibility.
## ⚠️ BREAKING ⚠️
The `[repository.editor] PREVIEWABLE_FILE_MODES` setting was removed.
This setting served no practical purpose and was not working correctly.
Instead a preview tab is always shown in the file editor when supported.
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Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
