labels added, custom cmaps added also if they are not in styles

[konstntokas]

Closes #1046 second part; see discussion comment and comments below

CHANGES.md will not be updated, as it is already updated in #1055.

Checklist:

• [x] Add unit tests and/or doctests in docstrings
• [ ] ~Add docstrings and API docs for any new/modified user-facing classes and functions~
• [x] New/modified features documented in docs/source/*
• [ ] ~Changes documented in CHANGES.md~
• [x] GitHub CI passes
• [x] AppVeyor CI passes
• [x] Test coverage remains or increases (target 100%)

[konstntokas]

• The labels are published now at the endpoint /colorbar.
• All color maps are published given in the section CustomColorMaps, even if they are not used in Styles