WIP: Change state to integer for issuables

What does this MR do?

Convert merge requests and issues state column to integer.

Warning!

This merge requests adds a migration which updates two big tables merge_requests and issues

• Before migration state column values: "opened" "closed" "merged" "locked"
• After migration state column values: 1 2 3 4

How is the migration working:

1. Creates a temporary column called new_state on each table
2. Migrates the new converted to integer values to the new column
3. Removes state column
4. Renames new_state to state
5. Rebuild related indexes with the new column

What are the relevant issue numbers?

closes https://gitlab.com/gitlab-org/gitlab-ce/issues/51789

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer