Previously the federation layer has written the empty string to the
database, now it writes nil/no value to it while our code still expects
the empty string in some cases. Restore the old assumption by returning
the empty string, the real raw value by now is available again in
StatusMessage#text for those that need it, such as validations or tests.

ignoring the error message

Previously it returned the return value of Logging::Logger#warn, which
is true for loggers that log the warn level. However
Diaspora::Federation::Receive::receive_relayable checks the return value
for truthiness in order to decide whether to attempt to relay it,
resulting in a NoMethodError: undefined method `parent' for
true:TrueClass in Diaspora::Federation::Receive::relay_relayable

This change is cosmetic as the exception raised prevented any action
that shouldn't happen anyway, so there's no actual logic change.

Send a second message to remote recipients with conversation-author
as salmon author if the conversation-author is local.

The first dispatch will fail on old pods. New pods will ignore the
second message, if the guid already exist.

the check that the participation parent author is local is now done in
the diaspora_federation gem

send retraction for relayable to target author if retracted by parent author

only deliver to parent author

message doesn't include Diaspora::Relayable anymore

also refactored relayable specs to use `let`

and delete old batch worker and receiver