disallow javascript: URI in the repository description

- Fixes an XSS that was introduced in
https://codeberg.org/forgejo/forgejo/pulls/1433
- This XSS allows for `href`s in anchor elements to be set to a
`javascript:` uri in the repository description, which would upon
clicking (and not upon loading) the anchor element execute the specified
javascript in that uri.
- [`AllowStandardURLs`](https://pkg.go.dev/github.com/microcosm-cc/bluemonday#Policy.AllowStandardURLs) is now called for the repository description
policy, which ensures that URIs in anchor elements are `mailto:`,
`http://` or `https://` and thereby disallowing the `javascript:` URI.
It also now allows non-relative links and sets `rel="nofollow"` on
anchor elements.
- Unit test added.

(cherry picked from commit bb448f3dc2)

modules/markup/sanitizer.go

@@ -179,6 +179,7 @@ func createDefaultPolicy() *bluemonday.Policy {
 // repository descriptions.
 func createRepoDescriptionPolicy() *bluemonday.Policy {
 	policy := bluemonday.NewPolicy()
+	policy.AllowStandardURLs()
 
 	// Allow italics and bold.
 	policy.AllowElements("i", "b", "em", "strong")