Label color improvements

This is a…

[X] Feature request
[ ] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report  
[ ] Documentation issue or request

Actually, the label colours have been consolidated to less colours, which is a good thing imo.

However, I think it goes a bit over the top for label groups which are supposed to be allowed multiple times (like group/ or notif/)

So my suggestion is:

• To use a single background colour for label groups where is only one element is supposed to be present (prio/, size/, …). Not sure why cat/feature is different from the scheme.
• For others, where are several possibilities allowed (cat/, group/, especially notif/ which targets different teams and its hard to depict from the colour to which team it belongs). For those I suggest to use different but similar colours (e.g. shades of green) and for notif/ totally separate colours.

[heiko-braun]

To add some background info, this is what inspired that changes:

The actual categories and colors are less important. what matters is the use of consistent color per category. The part that resonated with me was the red/green/blue separation for bugs/features/enhancements.

(Taken from https://robinpowered.com/blog/best-practice-system-for-organizing-and-tagging-github-issues/)

[rhuss]

To put the label “group/uxd” in a historical context, that label was meant at the time to be applied to issues and PRs which result in a UXD document stored within the syndesis repo. AFAIK, UXD artefacts are not stored anymore in the syndesis mono repo, so it would make sense to remove the “group/uxd” label.