Bug 1070001: Re-org Stylus - Activity

Moved .activity into its own file and changing class names to reflect component structure.

Also removed .actions reference from API key page (there were two different uses for the .actions class, but turns out it wasn't doing anything on API page, so re-styled that page to use a different component).

Pages to test (you will have to create some API keys):
https://developer-local.allizom.org/en-US/profiles/stephaniehobson
https://developer-local.allizom.org/en-US/users/account/keys