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/admin/repo/unadopted.tmpl

@@ -31,7 +31,7 @@
 										<div class="content">
 											<p>{{$.locale.Tr "repo.adopt_preexisting_content" $dir}}</p>
 										</div>
-										<form class="ui form" method="POST" action="{{AppSubUrl}}/admin/repos/unadopted">
+										<form class="ui form" method="post" action="{{AppSubUrl}}/admin/repos/unadopted">
 											{{$.CsrfTokenHtml}}
 											<input type="hidden" name="id" value="{{$dir}}">
 											<input type="hidden" name="action" value="adopt">
@@ -48,7 +48,7 @@
 										<div class="content">
 											<p>{{$.locale.Tr "repo.delete_preexisting_content" $dir}}</p>
 										</div>
-										<form class="ui form" method="POST" action="{{AppSubUrl}}/admin/repos/unadopted">
+										<form class="ui form" method="post" action="{{AppSubUrl}}/admin/repos/unadopted">
 											{{$.CsrfTokenHtml}}
 											<input type="hidden" name="id" value="{{$dir}}">
 											<input type="hidden" name="action" value="delete">