Make usage of Secret Service more consistent

In response to #630 (closed).

Right now this only corrects the unlocking of secret service collections.

Things that are planned and I'd appreciate some feedback on:

1. Add an additional type attribute to collection items, so that one can distinguish between passwords and access tokens
2. Remove the old password migration function, as it has been years since the rename
3. Instead of finding the appropriate item by iterating over all and finding the one with the correct label, persistently save information like UID, Matrix server domain, and identity server on disk and use them to find the secrets (they are not encrypted either way)

Of course these changes will need a migration routine of their own.