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/diff/section_split.tmpl

@@ -43,7 +43,7 @@
 					{{- $leftDiff := ""}}{{if $line.LeftIdx}}{{$leftDiff = $section.GetComputedInlineDiffFor $line $.root.locale}}{{end}}
 					{{- $rightDiff := ""}}{{if $match.RightIdx}}{{$rightDiff = $section.GetComputedInlineDiffFor $match $.root.locale}}{{end}}
 					<td class="lines-num lines-num-old del-code" data-line-num="{{$line.LeftIdx}}"><span rel="diff-{{$file.NameHash}}L{{$line.LeftIdx}}"></span></td>
-					<td class="lines-escape del-code lines-escape-old">{{if $line.LeftIdx}}{{if $leftDiff.EscapeStatus.Escaped}}<button class="toggle-escape-button btn interact-bg" title="{{template "repo/diff/escape_title" dict "diff" $leftDiff "locale" $.root.locale}}"></a>{{end}}{{end}}</td>
+					<td class="lines-escape del-code lines-escape-old">{{if $line.LeftIdx}}{{if $leftDiff.EscapeStatus.Escaped}}<button class="toggle-escape-button btn interact-bg" title="{{template "repo/diff/escape_title" dict "diff" $leftDiff "locale" $.root.locale}}"></button>{{end}}{{end}}</td>
 					<td class="lines-type-marker lines-type-marker-old del-code"><span class="gt-mono" data-type-marker="{{$line.GetLineTypeMarker}}"></span></td>
 					<td class="lines-code lines-code-old del-code">{{/*
 						*/}}{{if and $.root.SignedUserID $.root.PageIsPullFiles}}{{/*