This commit provides an alternative implementation
for storing tokens compared to the parent commit.
The idea is that we will not only need to store access
and refresh tokens obtained via Omniauth, but also the ones
to access third party services that will most likely be obtained
through OAuth 2.0 Token Exchange. This structure allows to store
all of these tokens in the same data model, while keeping the
implementation separated from the back-channel logout logic.
Jan Sandbrink
cdc304faf4