Add integration for Tap/Tab

[niknetniko]

This PR adds support for Tap and Tab, inspired by Tappb.

Current supported features:

• View all Tap products
• View all Tab transactions
• View account balance
• Create new Tab transactions
• Create new Tap orders (including barcode scanning of products)
• Manage credentials

This "Zeus mode" is hidden by default; you need to press Zeus in the about screen twice to enable it. Once enabled, access it from the navigation drawer or use the launcher shortcut (if your launcher supports it).

Most of the basics, error handling and UI are done. Some work remains:

• Fix a crash in the manual selector for Tap orders
• See if some testing can be done (Test Tab/Tap integration #569)
• Find a barcode scanning solution for the open variant of the app
• Fix linter, yet again

Future work:

• Autocomplete other people in Tab requests (requires API support)
• Show profile pictures of other people (requires API support)
• Better login method (manually specifying username, Tap API key and Tab API key is not needed, probably requires API support)
• Haldis? Maybe? Perhaps very difficult for the menu items, and not a lot of upsides
• Integrate with favourite items in Tap
• Allow shortcut to order favourite item directly
• ...?

Some screenshots: