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/admin/user/edit.tmpl

@@ -72,7 +72,7 @@
 				</div>
 				<div class="field {{if .Err_Website}}error{{end}}">
 					<label for="website">{{.locale.Tr "settings.website"}}</label>
-					<input id="website" name="website" type="url" value="{{.User.Website}}" placeholder="e.g. http://mydomain.com or https://mydomain.com" maxlength="255">
+					<input id="website" name="website" type="url" value="{{.User.Website}}" placeholder="http://mydomain.com or https://mydomain.com" maxlength="255">
 				</div>
 				<div class="field {{if .Err_Location}}error{{end}}">
 					<label for="location">{{.locale.Tr "settings.location"}}</label>
@@ -197,7 +197,7 @@
 		{{svg "octicon-trash"}}
 		{{.locale.Tr "settings.delete_account_title"}}
 	</div>
-	<form class="ui form" method="POST" action="{{.Link}}/delete">
+	<form class="ui form" method="post" action="{{.Link}}/delete">
 		<div class="content">
 			<p>{{.locale.Tr "settings.delete_account_desc"}}</p>
 			{{$.CsrfTokenHtml}}