UI: adjust rendering of main action button and dropdown button. (#41033)

https://mantis.ilias.de/view.php?id=41033

This includes following changes:

- remove margin from legacy SCSS file _component_forum.scss
- add 'il-item-actions' for alignment purposes to legacy SCSS file
  _component_forum.scss
- adjust the 'forum_threads_view' html template
- adjust the rendering of the main action button and the dropdown button
  in the 'ilObjForumGUI' file.

... instead of just changing the margin in the specific SCSS file.
With these changes the template will - at least partly - be build like
an UI Framework standard item template.