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).

docs/content/doc/development/hacking-on-gitea.en-us.md

@@ -48,6 +48,9 @@ Gitea uses `gofmt` to format source code. However, the results of
 recommended to install the version of Go that our continuous integration is
 running. As of last update, the Go version should be {{< go-version >}}.
 
+To lint the template files, ensure [Python](https://www.python.org/) and
+[Poetry](https://python-poetry.org/) are installed.
+
 ## Installing Make
 
 Gitea makes heavy use of Make to automate tasks and improve development. This