Fixes markdown rendering details where Gitea deviated from GitHub
unintentionally, and adds a `/devtest/markdown` page to review markdown
rendering.
- Paragraphs get the intended 1.5 line height
- Blockquotes, alerts and definition lists no longer inherit browser
default margins
- Loose task lists keep their spacing, the checkbox renders inside the
paragraph like on GitHub
- Footnote references render as `[1]` so adjacent ones stay apart
without CSS
- Code in headings, headings in summaries, nested ordered lists, tables,
`<mark>`, alert icons and `<hr>` follow GitHub
Co-authored-by: bircni <bircni@icloud.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
