Add new Rails/DefaultScope cop

[fatkodima]

Closes #76

Personally, I agree with everything stated in that issue and in numerous blog posts (including referenced in Reference), and do not find default_scope as useful, but as a time bomb and a source of bugs to be explored in production.

CI does not pass because there was a cop named DefaultScope in the past (but which did absolutely different thing, just checked default_scope style - lambda vs block).
https://github.com/rubocop-hq/rubocop/blob/423323cc7f5af33cd2123bfcadb8eec4da4e0d5e/lib/rubocop/config_obsoletion.rb#L66

So should that line be removed, or this cop renamed, or something else?

[bbatsov]

This was a long time ago, so I'd remove that line.

[andyw8]

I wonder if this can be phrased to indicate that using a named scope is not the only alternative. For example, hidden records could be moved to a separate table.

[fatkodima]

I think, most of the time the solution is just to name a scope. I'm fine with message, but maybe it is better to just remove the second sentence.

[Drenmi]

A convention I've come across a number of times is to always put default_scope at the top of the class to make it more discoverable. (After any includes.) Maybe this could be a configuration option?

[fatkodima]

@Drenmi
Albeit related to default_scope, but this cop is about a little different thing.
Maybe it is better to introduce new (or extend existing Layout/ClassStructure cop) to handle this and related?

[koic]

Thanks!