Add brand color to metadata API

[benpackard]

This would be a single set of r/g/b values at the top-level to use as a tint color on iOS labels and buttons throughout the app.

[chadalderson]

@benpackard - so you have a sense for what we are thinking with this approach:

And

[benpackard]

@landlessness Going to self-assign this one now since the values are in the API.

[landlessness]

cool

[benpackard]

For now the primary brand color is used sparingly on both the server's grouping title and for certain controls (e.g. buttons) on devices for that server. I think there is more that could be done with these but its a good first step. We can also easily support state-image colors using a similar approach (e.g. red locked door, blue bulb).