Adopt GNOME Online Accounts (goa)

[Letterus]

Problem

We're missing many account types and an easy way to set them up.

Proposal

GOA provides all of that. The xapp project forked the frontend. We could adopt and package that as an app as well and link it from switchboard (same as Mint does). We may later integrate it into switchboard as a module.

Prior Art (Optional)

https://github.com/xapp-project/gnome-online-accounts-gtk

[Letterus]

Read "We don't want GNOME Online Accounts. We will need our own credential portal indeed but that requires a lot of thought." over there: #209 (comment)

Some elaboration on the background would be great. Implementing all of this again looks like quite some work for a main feature.