Refactor issue sidebar and fix various problems (#37045)

Fix various legacy problems, including:

* Don't create default column when viewing an empty project
* Fix layouts for Windows
* Fix (partially) #15509
* Fix (partially) #17705

The sidebar refactoring: it is a clear partial-reloading approach,
brings better user experiences, and it makes "Multiple projects" /
"Project column on issue sidebar" feature easy to be added.

---------

Signed-off-by: wxiaoguang <wxiaoguang@gmail.com>

templates/repo/issue/view_content.tmpl

@@ -66,6 +66,7 @@
 			</div>
 
 			{{template "repo/issue/view_content/comments" .}}
+			<div class="timeline-item tw-hidden" id="timeline-comments-end"></div>
 
 			{{if and .Issue.IsPull (not $.Repository.IsArchived)}}
 				{{template "repo/issue/view_content/pull_merge_box".}}