Improve issue list filter (#24425)

Partial regression of #24393, not only regression, but broken for long
time, 24393 didn't really improve it but used wrong `overflow: scroll`.

Actually, that "ui secondary filter menu labels" shouldn't be set as
scrollable (I missed that at that time), the problem is: if a "ui menu"
has "dropdown" items, then it should not be scrollable. Otherwise the
dropdown menu can't be shown correctly.

And there are more problems:

* The "issue-filters" shouldn't be used anywhere else (copying&pasting
problem again ....)
* There is also an "issue-actions" container, it should also be fixed.
* There are similar problems on the milestone page.
* The old comment in code: "grid column" doesn't work well.

The major changes of this PR are: use "flex: 1" instead of "ui grid
column".

After this PR, not 100% perfect but much better than before.

templates/user/notification/notification_subscriptions.tmpl

@@ -11,7 +11,7 @@
 		</div>
 		<div class="ui bottom attached active tab segment">
 			{{if eq .Status 1}}
-				<div id="issue-filters" class="ui stackable grid">
+				<div class="ui stackable grid">
 					<div class="six wide column">
 						<div class="small-pill-buttons ui compact tiny menu">
 							<a class="{{if eq .State "all"}}active {{end}}item" href="{{$.Link}}?sort={{$.SortType}}&state=all&issueType={{$.IssueType}}&labels={{$.Labels}}">