Absolute positioned checkboxes overlay floated elements (#26870) (#27366)

Absolute positioned checkboxes  overlay floated elements (#26870) (#27366)

Backport #26870 by @rafh

Currently, checkboxes are positioned as absolute. This positioning
causes the input to overlay an element that has been floated within the
editor. Floated elements are useful if you want your text to wrap around
this element. This PR fixes the overlaying of checkboxes by removing the
absolute positioning, updating the `ul` padding, and
displaying`.task-list-item` `flex` to ensure inputs and the associated
label are on the same line.

Screenshots:

Before:
<img width="762" alt="Screenshot 2023-09-01 at 3 40 59 PM"
src="https://github.com/go-gitea/gitea/assets/6152817/570247c7-7f5c-4697-bfc9-ad4655e37991">

After:
<img width="762" alt="Screenshot 2023-09-01 at 3 42 20 PM"
src="https://github.com/go-gitea/gitea/assets/6152817/db53df45-1294-4eee-84c0-b21ac4fdf805">

Co-authored-by: Rafael Heard <rafael.heard@gmail.com>
Co-authored-by: rafh <rafaelheard@gmail.com>