Add css_classes support to formcheckbox

[stveit]

Turns out FormCheckBox needs to support css_classes to fully replace the old CheckBox.

This requires some change to how the templates are accessed. The changes in this PR makes it so the added argument css_classes is accessible in the html template. This could have been done the other way as well, but this is how the other classes that require css_classeswork.

I do also think its a bit less "magic" to define the template directly, instead of using a keyword "checkbox" that maps to the correct template in another file. Better to just define the correct template directly in the class.

[github-actions[bot]]

🦙 MegaLinter status: ✅ SUCCESS

Descriptor	Linter	Files	Fixed	Errors	Elapsed time
✅ PYTHON	black	992		0	11.14s
✅ PYTHON	ruff	987		0	0.11s

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Test results

    9 files      9 suites   8m 28s ⏱️ 2 139 tests 2 139 ✅ 0 💤 0 ❌ 4 017 runs  4 017 ✅ 0 💤 0 ❌

Results for commit 46fd948d.

:recycle: This comment has been updated with latest results.

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 60.39%. Comparing base (9db1bbf) to head (46fd948). Report is 6 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #3199 +/- ## ========================================== - Coverage 60.39% 60.39% -0.01% ========================================== Files 605 605 Lines 43703 43704 +1 Branches 48 48 ========================================== Hits 26395 26395 - Misses 17296 17297 +1 Partials 12 12 ```

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Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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