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.
---------
Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
