The Github bridge itself should not write anything. This commit removes
code writing to stdout and itroduces an event `ImportEventRateLimiting`
to `core.ImportResult` in order to inform about a rate limiting situation
of the Github GraphQL API. Now the communication with the user is
delegated to the various user interfaces.
The old implementation of the github bridge used maps to store several
channels holding data obtained from the Github API. Removing the maps and
simply packing data and channels together in a struct and passing it
through one single channel makes the program simpler in terms of
concurrency and, additionally, enables the garbage collector to free the
memory gradually without any additional provisions.
