Adds cop to enforce string limits on migrations

What does this MR do?

Adds cop to enforce string limits on migrations. This cop will analyze migrations that add columns with string, and report an offense if the string has no limit enforced.

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/64505

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Performance and testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Related to #64505 (closed)

To do

• Refactor and tests
• Disable current offenses