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).

Makefile

@@ -198,6 +198,7 @@ help:
 	@echo " - deps-frontend                    install frontend dependencies"
 	@echo " - deps-backend                     install backend dependencies"
 	@echo " - deps-tools                       install tool dependencies"
+	@echo " - deps-py                          install python dependencies"
 	@echo " - lint                             lint everything"
 	@echo " - lint-fix                         lint everything and fix issues"
 	@echo " - lint-actions                     lint action workflow files"
@@ -214,6 +215,7 @@ help:
 	@echo " - lint-css-fix                     lint css files and fix issues"
 	@echo " - lint-md                          lint markdown files"
 	@echo " - lint-swagger                     lint swagger files"
+	@echo " - lint-templates                   lint template files"
 	@echo " - checks                           run various consistency checks"
 	@echo " - checks-frontend                  check frontend files"
 	@echo " - checks-backend                   check backend files"
@@ -417,6 +419,10 @@ lint-editorconfig:
 lint-actions:
 	$(GO) run $(ACTIONLINT_PACKAGE)
 
+.PHONY: lint-templates
+lint-templates: .venv
+	@poetry run djlint $(shell find templates -type f -iname '*.tmpl')
+
 .PHONY: watch
 watch:
 	@bash build/watch.sh
@@ -893,7 +899,10 @@ deps-docs:
 	fi
 
 .PHONY: deps
-deps: deps-frontend deps-backend deps-tools deps-docs
+deps: deps-frontend deps-backend deps-tools deps-docs deps-py
+
+.PHONY: deps-py
+deps-py: .venv
 
 .PHONY: deps-frontend
 deps-frontend: node_modules
@@ -920,6 +929,10 @@ node_modules: package-lock.json
 	npm install --no-save
 	@touch node_modules
 
+.venv: poetry.lock
+	poetry install
+	@touch .venv
+
 .PHONY: npm-update
 npm-update: node-check | node_modules
 	npx updates -cu