Identity styles re-architecture

This PR follows a discussion with @phamann and @sndrs in #5434. That's a refactor I've been willing to do for a few weeks so that discussion was a good starting point.

- remove unused `theme/_colour.scss`
- segregate all styles related to identity into the `stylesheets/identity` folder
- break _identity.scss into _identity.scss and _formstack.scss
- break _disc.scss into _disc.scss and _activity-stream.scss

![ ](http://media.giphy.com/media/10jBPVms8GkWS4/giphy.gif)