Accent color value per space

## Is your feature request related to a problem? Please describe.
When there are many spaces, it may be hard to discern them quickly - especially in UIs with limited amount of space, or if users are not using cover images.

## Describe the solution you'd like
I'd like to be able to set and retrieve an accent color (RGB, alpha not needed) per space.

That accent color is quickly recognizable and can be used by clients for visual hints such as:
- rendering the default spaces icon in the accent color
- colored dots or squares in front of the space name in lists
- using the accent color to render a colorful header on top of a space
- adding color labels in Finder
- add a colored line below a space name in tab-based interfaces
- …

Teams / users could also use accent colors to visually "connect" related spaces (f.ex. different colors for R&D, controlling, administration, customer support, …) or to communicate that a certain rule set applies (f.ex. level of confidentiality - think green, yellow, red).

On creation, the server could randomly suggest a color value from a [cheerful, bright color palette](https://dribbble.com/shots/1353112-iOS7-Color-Palette) for each space.

## Additional context
That's the current state of spaces support in the iOS client.

It's still very much subject to change, but imagine what different accent colors for each space could do here to bring the UI to life:

Spaces list | Space 1 using OC brand colors | Space 2 using OC brand colors
--|--|--
 |  |