Add ability to customize admin UI (theming support)

[MrNaif2018]

Similar to bitcart/bitcart-store#319

We are taking the same approach: css variables.
It needs to be researched whether vuetify can generate those variables for css files to change colors at runtime

TODO:

• Implement initial theming support via vuetify's css vars support
• Make sure all external UI components also follow theme colors
• Load theme css file from theme_settings.admin_theme_url of the store, see Add theme_settings to stores bitcart#244
• Research for any vuetify analogues to bootswatch

Initially, at least customizing checkout page is our goal. Later whole admin UI could be made customizeable