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/diff/whitespace_dropdown.tmpl

@@ -12,7 +12,7 @@
 			<label class="gt-pointer-events-none">
 				<input class="gt-mr-3 gt-pointer-events-none" type="radio"{{if eq .WhitespaceBehavior "ignore-all"}} checked{{end}}>
 				{{.locale.Tr "repo.diff.whitespace_ignore_all_whitespace"}}
-				<label>
+			</label>
 		</a>
 		<a class="item" href="?style={{if .IsSplitStyle}}split{{else}}unified{{end}}&whitespace=ignore-change">
 			<label class="gt-pointer-events-none">