Db migrations for IDE OAuth

• gitlab-web-ide Spike MR
• gitlab Spike MR
  • Part 1 - Db migrations for IDE OAuth (!137212 - merged)
  • Part 2 - Add web_ide_oauth feature flag (!138015 - merged)

What does this MR do and why?

Db migrations for IDE OAuth. This adds a FK web_ide_oauth_application_id from application_settings to oauth_applications so that we can have a reference to the instance-wide OAuth application we'll use for the Web IDE.

• Please see this Spike MR for how we intend to populate this field !132496 (comment 1587293087)