rename devreg css to mkt

Just throwing this out there: earlier basta removed Marketplace consumer styles, ```css/mkt```, now that they are moved to Fireplace.

Now that:

- the name ```mkt``` isn't being used
- the name ```devreg```, where all of non-Commonplace Marketplace CSS files are stored, doesn't really mean anything

It would make sense just to move our CSS files into a folder named ```mkt```. I also renamed the ```devreg``` CSS bundle to ```devhub```. 

If this goes, JS would follow.