UX improvements

[shilangyu]

Various UX improvements to enhance the experience.

• [ ] When in the home tab, tapping the home tab icon should bring the user to the top
• [ ] When in the search tab, tapping the search tab icon should bring up the keyboard
• [ ] Community picker has autocomplete when creating a post
• [ ] Custom feeds can be created, similar to multireddits but expanded to include search terms and users
• [ ] Comments can be focused and threads can be followed in a new view

[testman42]

First one is what I miss the most. Also, should all of these be separate issues for easier tracking?