Provide option for a custom color palette

Description

The new navigation bar offers some predefined colors, however it doesn't allow yet to define custom colors, which are useful to identify the project of the GitLab instance you are in (to differentiate mainly from gitlab.com or other instances).

This would fix our top design issue we have at GNOME (together with https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15410) in our effort for a potential migration to GitLab.

Proposal

Proposal comes from @cperessini at https://gitlab.com/gitlab-org/gitlab-ce/issues/35012#note_41299626, and it's as follows:

image

Feature checklist

Make sure these are completed before closing the issue, with a link to the relevant commit.