UI Skinning

This PR introduces 2 new endpoints that can be used to have separate skins per tenant.

## How it works

There is a [LESS stylesheet](http://lesscss.org/) in the 3akai-ux folder at /shared/oae/css/oae.skin.less [1] which defines a bunch of variables that can be set by tenant admins. This file gets read, parsed and cached on server startup (or on changes in development mode).
Admins can then go into the Admin UI and update their skin by selecting the right colors, image paths, CSS values, .. The backend functionality to achieve this has been implemented (it re-uses the config api), the frontend functionality will follow shortly.

### The skin endpoint
Available at `http://tenant.oae.com/api/ui/skin`
It returns the actual CSS custom to the tenant. The CSS file will be generated once and cached for further requests.

### The variables endpoint(s)
Available at `http://tenant.oae.com/api/ui/skin/variables` and `http://admin.oae.com/api/ui/skin/variables`
It returns the variables that are declared in the less stylesheet and which can be set by (tenant) admins. These endpoints are only accessible by a global/tenant admin.

[1] I'll send a PR for this file to 3akai-ux shortly