Added created_at index to the members_click_events table

[vershwal]

Ref https://linear.app/tryghost/issue/ONC-216/improve-the-performance-of-the-membersevents-aggregated-click-event

[github-actions[bot]]

It looks like this PR contains a migration 👀 Here's the checklist for reviewing migrations:

General requirements

• [x] Satisfies idempotency requirement (both up() and down())
• [x] Does not reference models
• [x] Filename is in the correct format (and correctly ordered)
• [x] Targets the next minor version
• [x] All code paths have appropriate log messages
• [x] Uses the correct utils
• [x] Contains a minimal changeset
• [x] Does not mix DDL/DML operations

Schema changes

• [x] Both schema change and related migration have been implemented
• [ ] For index changes: has been performance tested for large tables
• [x] For new tables/columns: fields use the appropriate predefined field lengths
• [x] For new tables/columns: field names follow the appropriate conventions
• [x] Does not drop a non-alpha table outside of a major version

Data changes

• [x] Mass updates/inserts are batched appropriately
• [x] Does not loop over large tables/datasets
• [x] Defends against missing or invalid data
• [x] For settings updates: follows the appropriate guidelines

[vershwal]

We don't need this anymore!