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/issue/branch_selector_field.tmpl

@@ -1,7 +1,7 @@
 {{if and (not .Issue.IsPull) (not .PageIsComparePull)}}
 <input id="ref_selector" name="ref" type="hidden" value="{{.Reference}}">
 <input id="editing_mode" name="edit_mode" type="hidden" value="{{(or .IsIssueWriter .HasIssuesOrPullsWritePermission)}}">
-<form method="POST" action="{{$.RepoLink}}/issues/{{.Issue.Index}}/ref" id="update_issueref_form">
+<form method="post" action="{{$.RepoLink}}/issues/{{.Issue.Index}}/ref" id="update_issueref_form">
 	{{$.CsrfTokenHtml}}
 </form>