Code Hosts: update existing page and add modal for user tokens.

Created by: arussellsaw

Feature request description

We currently have a code hosts page in user settings, but it doesn't match the designs for our milestone. We should update it to fit. We also need to add the modal to accept user access tokens.

Designs here: https://www.figma.com/file/zXvqCASHDzmvKbrAl1EVsd/Public-%26-Private-Repos-for-Users-on-Cloud?node-id=563%3A21272

It's also using FilteredConnection here but probably doesn't need to, perhaps we can refactor the page a bit further to not need it.