Coding style: More rubocop rules & remove exclusions

[lentschi]

To prevent mixed coding styles we should add more rules as well as fix and then remove the exclusions made by .rubocop_todo.yml.

This issue should serve as a forum on what you think should be added as a rule.

[lentschi]

Style/HashSyntax to enforce { key: value } instead of { :key => value }

[RayOei]

It seems that using rubocop defaults is a good starting point?
It may be a sensible idea to work a bit through the code, as a newbie, and remove exclusions one-by-one.
Using autocorrect and modifying the rest manually file by file per exclusion.
The result(s) could be a PR per removed exclusion, for instance, to keep the number of changes in check?