Add migration to set default alert selector

[eradman]

What type of PR is this?

• Migration

Description

In commits fc1e1f7 and e44fcdb a new Selector option was added to alerts, which may be "first", "min" or "max". This migration sets the default to "first" for existing alerts.

How is this tested?

• Manually

docker compose exec server ./manage.py db downgrade 9e8c841d1a30
docker compose exec server ./manage.py db upgrade

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 64.01%. Comparing base (c08292d) to head (360e1f2).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7197   +/-   ##
=======================================
  Coverage   64.01%   64.01%           
=======================================
  Files         163      163           
  Lines       13410    13410           
  Branches     1905     1905           
=======================================
  Hits         8585     8585           
  Misses       4490     4490           
  Partials      335      335           

[eradman]

Closing in favor of #7475