Add developers.bumped_at

[joemasilotti]

This PR lays some groundwork for a change to the sorting algorithm.

It adds two new columns: developers.profile_updated_at and developers.bumped_at.

The former will be used to see how recent the profile was updated to know when to send the profile reminder emails. This means we can stop dancing around "touching" the profile when making changes outside of user-initiated flow.

The second is how devs will be sorted by default moving forward. Any public change to a profile will refresh the column if it was last updated more than 1 month ago.

In summary, a developer can "bump" their profile to the top of results once per month!

Pull request checklist

• My code contains tests covering the code I modified
• I linted and tested the project with bin/check
• I added significant changes and product updates to the changelog

To-do

• Ensure developers.profile_updated_at is being used for reminder emails
• Rip out any remaining touch: false calls or dances around touching records
• (New PR) Start sorting developers by the new column
• Announcement email and maybe a dedicated page to how sorting works

[joemasilotti]

Closed in favor of #856.