Use subdir for URL (#15446)

Fixes #15444

templates/repo/create.tmpl

 				<div class="ui attached segment">
 					{{template "base/alert" .}}
 
-					<p class="ui center">{{.i18n.Tr "repo.new_repo_helper" "/repo/migrate" | Safe}}</p>
+					<p class="ui center">{{.i18n.Tr "repo.new_repo_helper" (printf "%s%s" AppSubUrl "/repo/migrate") | Safe}}</p>
 
 					{{if not .CanCreateRepo}}
 						<div class="ui negative message">

templates/repo/issue/labels/label_list.tmpl

 				</div>
 				<div class="three wide column">
 					{{if $.PageIsOrgSettingsLabels}}
-						<a class="ui right open-issues" href="/issues?labels={{.ID}}">{{svg "octicon-issue-opened"}} {{$.i18n.Tr "repo.issues.label_open_issues" .NumOpenIssues}}</a>
+						<a class="ui right open-issues" href="{{AppSubUrl}}/issues?labels={{.ID}}">{{svg "octicon-issue-opened"}} {{$.i18n.Tr "repo.issues.label_open_issues" .NumOpenIssues}}</a>
 					{{else}}
 						<a class="ui right open-issues" href="{{$.RepoLink}}/issues?labels={{.ID}}">{{svg "octicon-issue-opened"}} {{$.i18n.Tr "repo.issues.label_open_issues" .NumOpenIssues}}</a>
 					{{end}}

templates/user/auth/activate.tmpl

 						{{end}}
 					{{else}}
 						{{if .NeedsPassword}}
-							<form class="ui form" action="/user/activate" method="post">
+							<form class="ui form" action="{{AppSubUrl}}/user/activate" method="post">
 								<div class="required inline field">
 									<label for="password">{{.i18n.Tr "password"}}</label>
 									<input id="password" name="password" type="password" autocomplete="off" required>