MissingForeignKeys detector assumptions

[fatkodima]

The checker makes an assumption that foreign-key like columns should be in the form of ..._id.

It can:

1. lead to false positives, when some table have some column ending with _id which is not a reference to other table, but, for example, some id from the external resource (users.stripe_id).
2. miss some custom named foreign keys: belongs_to :user, foreign_key: :user_key

I think, what should be really checked are models having non-polymorphic belongs_to associations without foreign keys.

Checking only models would make it easier to also ignore the case, when underlying models belong to different databases.