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/discord.tmpl

@@ -8,11 +8,11 @@
 		</div>
 		<div class="field">
 			<label for="username">{{.locale.Tr "repo.settings.discord_username"}}</label>
-			<input id="username" name="username" value="{{.DiscordHook.Username}}" placeholder="e.g. Gitea">
+			<input id="username" name="username" value="{{.DiscordHook.Username}}" placeholder="Gitea">
 		</div>
 		<div class="field">
 			<label for="icon_url">{{.locale.Tr "repo.settings.discord_icon_url"}}</label>
-			<input id="icon_url" name="icon_url" value="{{.DiscordHook.IconURL}}" placeholder="e.g. https://example.com/assets/img/logo.svg">
+			<input id="icon_url" name="icon_url" value="{{.DiscordHook.IconURL}}" placeholder="https://example.com/assets/img/logo.svg">
 		</div>
 		{{template "repo/settings/webhook/settings" .}}
 	</form>