Fix required checkboxes in issue forms (#27592)

Fix required checkboxes in issue forms (#27592)

If you set a checkbox as required in a issue form at the moment, the
checkbox is checked and read only, what does not make much sense. With
this PR, the Checkbox actually needs to be checked. The label supports
now also Markdown. This matches GitHub's behaviour.

And yes, I know the CSS is a ugly workaround. It looks like the given
CSS code is part Fomantic and I don't know how to change that. The
Maintainers are free to change that.

![grafik](https://github.com/go-gitea/gitea/assets/15185051/3f35be75-b0b4-42a7-9048-a4970384a035)