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/install.tmpl

@@ -139,7 +139,7 @@
 					</div>
 					<div class="inline required field">
 						<label for="domain">{{.locale.Tr "install.domain"}}</label>
-						<input id="domain" name="domain" value="{{.domain}}" placeholder="e.g. try.gitea.io" required>
+						<input id="domain" name="domain" value="{{.domain}}" placeholder="try.gitea.io" required>
 						<span class="help">{{.locale.Tr "install.domain_helper"}}</span>
 					</div>
 					<div class="inline field">
@@ -154,7 +154,7 @@
 					</div>
 					<div class="inline required field">
 						<label for="app_url">{{.locale.Tr "install.app_url"}}</label>
-						<input id="app_url" name="app_url" value="{{.app_url}}" placeholder="e.g. https://try.gitea.io" required>
+						<input id="app_url" name="app_url" value="{{.app_url}}" placeholder="https://try.gitea.io" required>
 						<span class="help">{{.locale.Tr "install.app_url_helper"}}</span>
 					</div>
 					<div class="inline required field">