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/packagist.tmpl

@@ -4,15 +4,15 @@
 		{{.CsrfTokenHtml}}
 		<div class="required field {{if .Err_Username}}error{{end}}">
 			<label for="username">{{.locale.Tr "repo.settings.packagist_username"}}</label>
-			<input id="username" name="username" value="{{.PackagistHook.Username}}" placeholder="e.g. Gitea" autofocus required>
+			<input id="username" name="username" value="{{.PackagistHook.Username}}" placeholder="Gitea" autofocus required>
 		</div>
 		<div class="required field {{if .Err_APIToken}}error{{end}}">
 			<label for="api_token">{{.locale.Tr "repo.settings.packagist_api_token"}}</label>
-			<input id="api_token" name="api_token" value="{{.PackagistHook.APIToken}}" placeholder="e.g. X5F_tZ-Wj3c1vqaU2Rky" required>
+			<input id="api_token" name="api_token" value="{{.PackagistHook.APIToken}}" placeholder="X5F_tZ-Wj3c1vqaU2Rky" required>
 		</div>
 		<div class="required field {{if .Err_PackageURL}}error{{end}}">
 			<label for="package_url">{{.locale.Tr "repo.settings.packagist_package_url"}}</label>
-			<input id="package_url" name="package_url" value="{{.PackagistHook.PackageURL}}" placeholder="e.g. https://packagist.org/packages/laravel/framework" required>
+			<input id="package_url" name="package_url" value="{{.PackagistHook.PackageURL}}" placeholder="https://packagist.org/packages/laravel/framework" required>
 		</div>
 		{{template "repo/settings/webhook/settings" .}}
 	</form>