Detect markdown line width, resolve relative URLs (#332)

~~this is semi-blocked by https://github.com/charmbracelet/glamour/pull/96, but behaviour isn't really worse than the previous behaviour (most links work, some are still broken)~~

#### testcase for link resolver
```
tea pr 332
tea checkout 332 && make install && tea pr 332
```

- [rel](./332)
- [abs](/gitea/tea/pulls/332)
- [full](https://gitea.com/gitea/tea/pulls/332)

Co-authored-by: Norwin Roosen <git@nroo.de>
Co-authored-by: 6543 <6543@obermui.de>
Reviewed-on: https://gitea.com/gitea/tea/pulls/332
Reviewed-by: 6543 <6543@obermui.de>
Reviewed-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: Norwin <noerw@noreply.gitea.io>
Co-committed-by: Norwin <noerw@noreply.gitea.io>

vendor/github.com/microcosm-cc/bluemonday/helpers.go

@@ -135,7 +135,7 @@ func (p *Policy) AllowStandardURLs() {
 	// Most common URL schemes only
 	p.AllowURLSchemes("mailto", "http", "https")
 
-	// For all anchors we will add rel="nofollow" if it does not already exist
+	// For linking elements we will add rel="nofollow" if it does not already exist
 	// This applies to "a" "area" "link"
 	p.RequireNoFollowOnLinks(true)
 }