Add context when rendering labels or emojis (#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: Kyle D <kdumontnu@gmail.com>

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>

templates/repo/issue/labels/label_list.tmpl

@@ -31,8 +31,8 @@
 			<li class="item">
 			<div class="ui grid middle aligned">
 				<div class="nine wide column">
-					{{RenderLabel .}}
-					{{if .Description}}<br><small class="desc">{{.Description | RenderEmoji}}</small>{{end}}
+					{{RenderLabel $.Context .}}
+					{{if .Description}}<br><small class="desc">{{.Description | RenderEmoji $.Context}}</small>{{end}}
 				</div>
 				<div class="four wide column">
 					{{if $.PageIsOrgSettingsLabels}}
@@ -70,8 +70,8 @@
 					<li class="item">
 					<div class="ui grid middle aligned">
 						<div class="nine wide column">
-							{{RenderLabel .}}
-							{{if .Description}}<br><small class="desc">{{.Description | RenderEmoji}}</small>{{end}}
+							{{RenderLabel $.Context .}}
+							{{if .Description}}<br><small class="desc">{{.Description | RenderEmoji $.Context}}</small>{{end}}
 						</div>
 						<div class="four wide column">
 							<a class="ui left open-issues" href="{{$.RepoLink}}/issues?labels={{.ID}}">{{svg "octicon-issue-opened"}} {{$.locale.Tr "repo.issues.label_open_issues" .NumOpenRepoIssues}}</a>