Enables frozens string for the following: * lib/gitlab/conflict/**/*.rb * lib/gitlab/cross_project_access/**/*.rb * lib/gitlab/cycle_analytics/**/*.rb * lib/gitlab/data_builder/**/*.rb * lib/gitlab/database/**/*.rb * lib/gitlab/dependency_linker/**/*.rb * lib/gitlab/diff/**/*.rb * lib/gitlab/downtime_check/**/*.rb * lib/gitlab/email/**/*.rb * lib/gitlab/etag_caching/**/*.rb Partially addresses gitlab-org/gitlab-ce#47424.
Starting with GitLab 9.1.0 we will no longer allow downtime migrations unless absolutely necessary. This commit updates the various developer guides and adds code that is necessary to make zero downtime migrations less painful.
