net: Break disconnecting out of Ban()

Summary:
These are separate events which need to be carried out by separate subsystems.

This also cleans up some whitespace and tabs in qt to avoid getting flagged by
the linter.

Current behavior is preserved.

This is extracted from Core PR14605 : https://github.com/bitcoin/bitcoin/pull/14605/commits/7cc2b9f6786f9bc33853220551eed33ca6b7b7b2

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4069