WIP: Allow plugins to register themes

[joaquimrocha]

This is a WIP PR for registering themes from plugins.
It's still very rough, but it's working...

fixes #2150

How to test:

1. Go to plugins/headlamp-plugin and run npm run build
2. Then run npm pack
3. Then go to the new plugins/examples/theme and install the headlamp-plugin we packed there: npm i ../../headlamp-plugin/kinvolk-headlamp-plugin...tar.gz
4. Run the plugin with npm i ; npm run start
5. Go to the Headlamp settings to see new themes

Progress:

• Add proper typing for themes
• Abstract themes into their own definition/module
• Sanitize the themes to not have stuff like somethingBg but also something: { background: ... }
• Replace the primaryColor usage by something more appropriate
• Allow plugins to register themes
• Allow to change the theme from settings
• Create a proper theme button which previews the colors for the theme
• Allow to associate a logo with the theme

[joaquimrocha]

I am fine if the team continues this PR BTW.

[joaquimrocha]

FTR, @sniok had suggested we go with a thinner version of the theme that plugins may override for now. I agree with that and the Design team is working on getting us these higher level theme concepts for us to set up and share in our theme and for the plugins to override.