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/base/head_navbar.tmpl

@@ -91,7 +91,7 @@
 							{{if .ActiveStopwatch}}{{Sec2Time .ActiveStopwatch.Seconds}}{{end}}
 						</span>
 					</a>
-					<form class="stopwatch-commit" method="POST" action="{{.ActiveStopwatch.IssueLink}}/times/stopwatch/toggle">
+					<form class="stopwatch-commit" method="post" action="{{.ActiveStopwatch.IssueLink}}/times/stopwatch/toggle">
 						{{.CsrfTokenHtml}}
 						<button
 							type="submit"
@@ -99,7 +99,7 @@
 							data-tooltip-content="{{.locale.Tr "repo.issues.stop_tracking"}}"
 						>{{svg "octicon-square-fill"}}</button>
 					</form>
-					<form class="stopwatch-cancel" method="POST" action="{{.ActiveStopwatch.IssueLink}}/times/stopwatch/cancel">
+					<form class="stopwatch-cancel" method="post" action="{{.ActiveStopwatch.IssueLink}}/times/stopwatch/cancel">
 						{{.CsrfTokenHtml}}
 						<button
 							type="submit"