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/aymerick/douceur/css/stylesheet.go

@@ -0,0 +1,25 @@
+package css
+
+// Stylesheet represents a parsed stylesheet
+type Stylesheet struct {
+	Rules []*Rule
+}
+
+// NewStylesheet instanciate a new Stylesheet
+func NewStylesheet() *Stylesheet {
+	return &Stylesheet{}
+}
+
+// Returns string representation of the Stylesheet
+func (sheet *Stylesheet) String() string {
+	result := ""
+
+	for _, rule := range sheet.Rules {
+		if result != "" {
+			result += "\n"
+		}
+		result += rule.String()
+	}
+
+	return result
+}