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)
You're right about the `+` and `-`, it's better to add space around them to avoid these issues. I actually started adding space around `*` and `/` in this PR, and then [reverted the spacing tweaks](8876ef002ec94751b1bfb793b32cfd2eb2f303d0), because it should be a separate PR as you say.