Expose textcolor from public labels API

Problem to solve

The text color for labels is calculated by the backend based on the background color:

Our internal, undocumented API (https://gitlab.com/gitlab-org/gitlab-ce/labels.json) contains the text_color of a label but the official one (https://gitlab.com/api/v4/projects/gitlab-org%2Fgitlab-ce/labels.json) doesn't.

For consistency, we may also want to include it in the autocompletion endpoint (https://gitlab.com/gitlab-org/gitlab-ce/autocomplete_sources/labels.json).

Target audience

Not quite sure—it would allow third party applications (including https://docs.gitlab.com/) to render labels correctly.

Proposal

Probably just adding text_color to https://gitlab.com/gitlab-org/gitlab-ce/blob/25af9032750c215860829fecb196da1e1c5ace6b/lib/api/entities.rb#L1006.