Fix detecting nested EE constants in RuboCop

See merge request gitlab-org/gitlab-ce!24430

The InjectEnterpriseEditionModule cop would not detect certain nested EE
constants such as `EE::Foo::Bar::Baz`. This could result in it not
enforcing `prepend` being placed on the last line. This commit fixes
this by just performing a string match on the line, instead of relying
on AST matching.

The Background migration specs also run against a certain version of
the database. So we should avoid using factories.

This Cop enforces the rule that injecting EE modules (using prepend,
include, or extend) is done by placing the injection on the last line of
a file, instead of somewhere in the middle. By placing these lines at
the very end, merge conflicts will not happen.

It might happen you want to make the reference column have a unique
value, or you want to create partial indexes. So instead of only
accepting a `true` value, also accept a hash of options.

This cop forbids the use of `element :foo, 'pattern'` and
`element :bar, /pattern/` in QA files.

Signed-off-by: Rémy Coutable <remy@rymai.me>