Sticky buttons

[math-GH]

Inspired by #6014
Pre-work done by #6202

Keep the submit button always reachable.

have a look onto the logs

have a look onto the feed configuration

How to test the feature manually:

1. go to display option and enable the sticky buttons (last option)
2. see the sticky buttons at the bottom of window

Pull request checklist:

• clear commit messages
• code manually tested

[Alkarex]

I think we should just make the new behaviour the default, without new option (I would like to limit the number of options, as it increases the amount of tests and translations and complexity)