Add floating post-status button on mobile

Adds a material style floating action button for posting new statuses.

• Pressing the button opens a modal for posting new status.
• The button moves out of the way when you scroll down and appears again when you scroll up.
• The draft doesn't get wiped when you close the modal by tapping outside and when you navigate.
• The modal can be scrolled when the status you're writing on gets long, and when you have attachments taking up space too, there's guaranteed empty space for closing above and below the scrollable modal.
• Added slight fade to modal backgrounds across the application.
• Removed the "Post new status" from burg menu
• Posting makes the modal close automatically

Things to potentially improve later on:

• Better theming support? I played it safe by making it use the plain button color now, but it can look kinda off. Link color is not guaranteed to work on so many themes.
• Somehow stop scrolling behind the modal while keeping scrolling on the modal? I don't know a good way of achieving this so I just left it as it is, it's not a dealbreaker.

(Ignore my browser's weird rendering errors in the rounded corners)