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).

pyproject.toml

@@ -0,0 +1,19 @@
+[tool.poetry]
+name = "gitea"
+version = "0.0.0"
+description = ""
+authors = []
+
+[tool.poetry.dependencies]
+python = "^3.8"
+
+[tool.poetry.group.dev.dependencies]
+djlint = "1.31.0"
+
+[tool.djlint]
+profile="golang"
+ignore="H005,H006,H008,H013,H014,H016,H020,H021,H023,H026,H030,H031,T027"
+
+[build-system]
+requires = ["poetry-core"]
+build-backend = "poetry.core.masonry.api"