Customize color theme

Proposal

Allow the user to customize their GitLab theme with a color of their choice.

• Replace existing theme picker with a color picker with input for custom color.
• Validate and persist valid custom color as the main theme-color
• Calculate an appropriate color for foreground color
  • If background color is dark, then foreground color should be light (vice versa)
  • Contrast ratio of background and foreground should meet AA standards (4.5:1)
• Optional: Change section heading to Navigation appearance
• Stretch: Allow user to select the foreground color as well

Current	Proposal

Figma: https://www.figma.com/file/fluIEruqeRxv3jx9B5oxBM/Customize-theme?node-id=1%3A3