Refactor Events action constants into Rails enum method

Summary

"Event class needs refactoring to leverage Rails enums. It will simplify the code significantly." - #7168 (comment 327105946)

Improvements

Consolidate Event model's various action constants into a single call to Rails enum, and make any necessary changes to accommodate that.

Note: The events#action column is already correctly configured as a SMALLINT in the database, so this is purely a Ruby change.

Risks

Minimal risks. Rails enum attributes are more flexible, not less.

Involved components

• app/models/event.rb

Optional: Intended side effects

• Less code
• More flexibility in writing code