Refactor ivan's code (aa4b684d0682ff9faf1577653fa5ceabbc6e0f20) to remove the
time.sleep.

- Resolves: #7414

If the user wants to register a new account we check whether the
provider allows registration or not right after getting the
provider.json file and show an error msg on the wizard if not allowed.

Also, there is a new signal to handle the error raised by the server if
a registration attempt is made but is rejected with error 403.

- Resolves: #6594

Is valid for a provider not to provide locations for their gateways.

- Resolves: #7281

Replace logging.getLogger with custom get_logger.
Remove some unneeded dependencies and reorder other.

The backend was unsetting the soledad proxied object before actually calling
the soledad close() method and this was causing the application to hang when
exitting.

As the local soledad component's cancel_bootstrap() method is called before
the close() method, we'd better not unset the proxied object there, and let
only the close method do it. This commit fixes this by just removing the line
that unsets the proxied object in the wrong place.

Before this commit, the backend used plain pyzmq bindings for communicating
with the frontend. This implements the txzmq twisted-powered bindings instead.

Closes: #6360

... on them, to user-writeable only.

I think we could consider ipc for the curve-case too, at least for the
platforms that support them.

Use global flag for ZMQ_HAS_CURVE.
Closes #6646

This closes: #6658 and #6691