Fix prompted auth storage
It turns out that the prompted auth storage locations for iOS (a lot) and android (a little) are not that reliable. We have had multiple iOS users and at least one android user have their stored token deleted.
One of the iOS users said that she had problems with her phone so she had reset to defaults. Maybe that is the problem?
We have seen this for only one android user so far, but it appears to happen on android as well. Storing this in the database instead since that appears to be more reliable.
This fixes
e-mission/e-mission-docs#579