Start failover procedure

Requires !159 (merged)

Compare with https://gitlab.com/gitlab-com/migration/compare/scripts...start-failover-procedure

Simple script for setting up the migration Failover Execution issues:

Preparing for a Failover Run

1. Setup bin/source_vars: cp ./bin/source_vars_template.sh ./bin/source_vars
2. Configure bin/source_vars: The variables are explained in the file. Since this contains secrets, this file should not be checked in. (it's .gitignore'd)
3. Setup the workflow issues": Run bin/start-failover-procedure.sh. This will setup several issues in the issue tracker for performing the checks, failover, tests, etc.
   • Any variables in the template in the format __VARIABLE__ will be substituted with their values from the bin/source_vars file, saving manual effort.

cc @nick.thomas @digitalmoksha