QA: Ignore RuboCop rules per the coding conventions

Ignore Metrics/ClassLength and other rules per our Ruby coding
conventions.

https://www.mediawiki.org/w/index.php?title=Manual:Coding_conventions/Ruby&oldid=1280750

Change-Id: I8f3486bc021c8d7e91274545122368af7abde97e
Merge "QA: Ignore RuboCop rules per the coding conventions"