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)
I think it would be a good one to enforce. i think it’s a fault of scss that you can have

```scss
$x-1: 10;
$x-2: $x-1+1;
```
but not 
```scss
$x-1: 10;
$x-2: $x-1-1;
```
neither should be ok imo. although it’s permitted, i think it makes for easier reading to disallow it?

but if a lot of the codebase is written this way then I agree that’s a separate PR.