Use submodules in review apps

Description

• Updates ops/start_review.sh to clone with --recursive so that review apps have access to the testdata
  • enables use of testdata branches in the review app
• Modifies review_build CI stage to take better advantage of cached docker images and not constantly rebuild from scratch
• Fixes checking out branches of testdata in triggered pipelines (see also: ella-frontend!159 (merged))

Related issues

Notes to review (code/docs/QA)

Tests

General

• Tests have been added that prove my fix is effective or that my feature works
• Related tests have been modified/removed

Hypothesis testing:

• Soak testing has been done
• Distribution between positive / negative cases has been checked

Database

• Includes changes to database schema
• Includes necessary database migrations

Configuration

• Includes changes to configuration
• Includes configuration migration instructions in documentation