New user option: Always show floating New Post button

The issue: Currently on the desktop site, if you're scrolled down the sidebar reading your notifications and want to post a new status, you have to leave your position to scroll back up. This seems like an unneccessary restriction given that the mobile post button and the modal for it exist and these work perfectly fine on the desktop version when not hidden by pleroma-fe.

The code: What this does is add a new option called "Always show floating New Post button" in the Compose section of the General settings tab. When enabled, pleroma-fe will no longer hide the New Post button. To prevent overlap with the Shoutbox, when this option enabled the Shoutbox will be moved to the left side instead of the right using a class and some CSS.

Issues with this: None known, this isn't adding a new component or anything but merely letting users access a feature they already have but was previously only accessible in the mobile nav.

Changelog and contributors file updated as well, and tested with npm run dev as well as on my instance (screens are from my instance)

Screens: