Refactor push rules - create additional table

What does this MR do?

As we want to introduce group push rules and not introduce any technical debt and duplication, according to discussion here: !25279 (comment 293644035) here I did following changes:

• adding project_push_rule table
• adding ProjectPushRule model that is connects project and push rule
• adding storing ProjectPushRule when new push rule for project is created
• adding target_type to push rule table

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability 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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team