Issue/PR Context Popups (#9822)

* Add data-index attribute to issue anchors

Signed-off-by: jolheiser <john.olheiser@gmail.com>

* Init JS

Signed-off-by: jolheiser <john.olheiser@gmail.com>

* Add required data to anchor

Signed-off-by: jolheiser <john.olheiser@gmail.com>

* Finish popup

Signed-off-by: jolheiser <john.olheiser@gmail.com>

* Revert changes to html.go

Signed-off-by: jolheiser <john.olheiser@gmail.com>

* Better octicon contexts

Signed-off-by: jolheiser <john.olheiser@gmail.com>

* Split out popup function for re-use

Signed-off-by: jolheiser <john.olheiser@gmail.com>

* Style changes, test fixes, and cross-reference support

Signed-off-by: jolheiser <john.olheiser@gmail.com>

* Prefer em to px

Signed-off-by: jolheiser <john.olheiser@gmail.com>

* Move label margin to base CSS

Signed-off-by: jolheiser <john.olheiser@gmail.com>

* Move JS to separate file.

Signed-off-by: jolheiser <john.olheiser@gmail.com>

* Move JS to features and fix module

Signed-off-by: jolheiser <john.olheiser@gmail.com>

* Remove query-string and hash

Co-Authored-By: silverwind <me@silverwind.io>

Co-authored-by: Lauris BH <lauris@nix.lv>
Co-authored-by: Antoine GIRARD <sapk@users.noreply.github.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: zeripath <art27@cantab.net>

modules/markup/sanitizer.go

@@ -53,6 +53,9 @@ func ReplaceSanitizer() {
 	// Allow <kbd> tags for keyboard shortcut styling
 	sanitizer.policy.AllowElements("kbd")
 
+	// Allow classes for anchors
+	sanitizer.policy.AllowAttrs("class").Matching(regexp.MustCompile(`ref-issue`)).OnElements("a")
+
 	// Custom keyword markup
 	for _, rule := range setting.ExternalSanitizerRules {
 		if rule.Regexp != nil {