Fix workspaces color

[LivOriona]

Always have the same colour of the workspace label, even after reloading the page, on all pages displaying the workspace:

• workspace management interface (a),

• list of workspaces in the left-hand menu (b),

• list of workspaces in the main workspace page (c)

• Give the possibility to choose the workspace color at the creation of the workspace.
  
  

[StCyr]

Implemented in #75 just missing persistent storage in DB

[LivOriona]

The rest of this issue should be dealt with in Sprint 9:

1. the colour of a Workspace must be the same throughout the Workspace application (see points (a)(b)(c) of the first message of this issue)
2. The possibility to choose the colour of a Workspace is made. Improvement: add an eyedropper icon when hovering over the colour of a Workspace in its management interface, to indicate to the user that this parameter can be modified (see Clémentine's mock-ups, slides 10 and 11 of the Sprint 8 demo)

[zak39]

The color is persist now in the #109 PR