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/settings/lfs_locks.tmpl

@@ -5,7 +5,7 @@
 				<a href="{{.LFSFilesLink}}">{{.locale.Tr "repo.settings.lfs"}}</a> / {{.locale.Tr "repo.settings.lfs_locks"}} ({{.locale.Tr "admin.total" .Total}})
 			</h4>
 			<div class="ui attached segment">
-				<form class="ui form ignore-dirty" method="POST">
+				<form class="ui form ignore-dirty" method="post">
 					{{$.CsrfTokenHtml}}
 					<div class="ui fluid action input">
 						<input name="path" value="" placeholder="{{.locale.Tr "repo.settings.lfs_lock_path"}}" autofocus>
@@ -37,7 +37,7 @@
 							</td>
 							<td>{{TimeSince .Created $.locale}}</td>
 							<td class="right aligned">
-								<form action="{{$.LFSFilesLink}}/locks/{{$lock.ID}}/unlock" method="POST">
+								<form action="{{$.LFSFilesLink}}/locks/{{$lock.ID}}/unlock" method="post">
 									{{$.CsrfTokenHtml}}
 									<button class="ui primary button"><span class="btn-octicon">{{svg "octicon-lock"}}</span>{{$.locale.Tr "repo.settings.lfs_force_unlock"}}</button>
 								</form>