This version of remote config allows non-boolean values to be returned
to clients but unfortunately limits the configuration to only one
value or another. There is no way to configure more than two values
for the same key with this setup.
In preparation for resumable uploads, this creates a separate
attachment authorization endpoint that creates a signed URL for
accessing GCP Storage through Signal's CDN2. This should allow Signal
clients to do byte-level resume of media uploads.
1) Rename "active" methods to be "enabled," since they aren't
really about "activity."
2) Make authentication fail if a device or account is in dissabled
state.
3) Let some controllers authenticate accounts that are in a
disabled state.
