Implement issue and pull request templates.

Similar to GitHub:
https://github.com/blog/2111-issue-and-pull-request-templates

Priority:
- root
- .gogs
- .github

templates/repo/issue/comment_tab.tmpl

@@ -4,7 +4,7 @@
 		<a class="item" data-tab="preview" data-url="{{AppSubUrl}}/api/v1/markdown" data-context="{{.RepoLink}}">{{.i18n.Tr "repo.release.preview"}}</a>
 	</div>
 	<div class="ui bottom attached active tab segment" data-tab="write">
-		<textarea id="content" name="content" tabindex="4"></textarea>
+		<textarea id="content" name="content" tabindex="4">{{if .IssueTemplate}}{{.IssueTemplate}}{{end}}{{if .PullRequestTemplate}}{{.PullRequestTemplate}}{{end}}</textarea>
 	</div>
 	<div class="ui bottom attached tab segment markdown" data-tab="preview">
 		{{.i18n.Tr "repo.release.loading"}}