Fix problems with the proxy tab and `governance_operations.scss` file

Requires: !284 (merged)

This MR resolves two issues noticed on the proxy tab when the above MR was tested and one noticed on proposals tab:

1. When a user changed proxy and quickly went to the 'management' tab, the screen did not catch that a new proxy had appeared and displayed the old proxy status.
2. When the user tried to set the proxy to self, the application crashed because create_operation returned None and ValueError was raised because the application tried to retrieve the operation from the create_operations method, which was not valid.
3. The .scss structure should be better - now governance_operations.scss overwrites some properties - it shouldn't.