Add context when rendering labels or emojis (#23281) (#23319)

Backport #23281

This branch continues the work of #23092 and attempts to rid the
codebase of any `nil` contexts when using a `RenderContext`.

Anything that renders markdown or does post processing may call
`markup.sha1CurrentPatternProcessor()`, and this runs
`git.OpenRepository()`, which needs a context. It will panic if the
context is `nil`. This branch attempts to _always_ include a context
when creating a `RenderContext` to prevent future crashes.

Co-authored-by: Jonathan Tran <jon@allspice.io>

templates/repo/issue/labels/label.tmpl

@@ -3,5 +3,5 @@
 	id="label_{{.label.ID}}"
 	href="{{.root.RepoLink}}/{{if or .root.IsPull .root.Issue.IsPull}}pulls{{else}}issues{{end}}?labels={{.label.ID}}"{{/* FIXME: use .root.Issue.Link or create .root.Link */}}
 >
-	{{RenderLabel .label}}
+	{{RenderLabel $.Context .label}}
 </a>