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/webhook/base_list.tmpl

@@ -50,7 +50,7 @@
 			<div class="item truncated-item-container">
 				<span class="text {{if eq .LastStatus 1}}green{{else if eq .LastStatus 2}}red{{else}}grey{{end}} gt-mr-3">{{svg "octicon-dot-fill" 22}}</span>
 				<a class="text truncate gt-f1 gt-mr-3" title="{{.URL}}" href="{{$.BaseLink}}/{{.ID}}">{{.URL}}</a>
-				<a class="muted gt-p-3" href="{{$.BaseLink}}/{{.ID}}">{{svg "octicon-pencil"}}</a></span>
+				<a class="muted gt-p-3" href="{{$.BaseLink}}/{{.ID}}">{{svg "octicon-pencil"}}</a>
 				<a class="delete-button gt-p-3" data-url="{{$.Link}}/delete" data-id="{{.ID}}">{{svg "octicon-trash"}}</a>
 			</div>
 		{{end}}