Add template linting via djlint (#25212)

So I found this [linter](https://github.com/Riverside-Healthcare/djlint)
which features a mode for go templates, so I gave it a try and it did
find a number of valid issue, like unbalanced tags etc. It also has a
number of bugs, I had to disable/workaround many issues.

Given that this linter is written in python, this does add a dependency
on `python` >= 3.8 and `poetry` to the development environment to be
able to run this linter locally.

- `e.g.` prefixes on placeholders are removed because the linter had a
false-positive on `placeholder="e.g. cn=Search"` for the `attr=value`
syntax and it's not ideal anyways to write `e.g.` into a placeholder
because a placeholder is meant to hold a sample value.
- In `templates/repo/settings/options.tmpl` I simplified the logic to
not conditionally create opening tags without closing tags because this
stuff confuses the linter (and possibly the reader as well).

templates/repo/issue/view_content/comments_delete_time.tmpl

@@ -3,7 +3,7 @@
 		{{if (or .ctxData.IsAdmin (and .ctxData.IsSigned (eq .ctxData.SignedUserID .comment.PosterID)))}}
 			<span class="ui float right">
 				<div class="ui mini modal issue-delete-time-modal" data-id="{{.comment.Time.ID}}">
-					<form method="POST" class="delete-time-form" action="{{.ctxData.RepoLink}}/issues/{{.ctxData.Issue.Index}}/times/{{.comment.TimeID}}/delete">
+					<form method="post" class="delete-time-form" action="{{.ctxData.RepoLink}}/issues/{{.ctxData.Issue.Index}}/times/{{.comment.TimeID}}/delete">
 						{{.ctxData.CsrfTokenHtml}}
 					</form>
 					<div class="header">{{.ctxData.locale.Tr "repo.issues.del_time"}}</div>