Move COM_URL related variables to a module

Summary

Move COM_URL related variables in lib/gitlab.rb to a module which may be overridden in other variants of GitLab

Improvements

Other variants of GitLab could override the methods of various links related configurations in their own folder instead of hard updating. Changing the COM_URL directly will cause codebase conflicts.

See issue gitlab-jh/gitlab#51 (closed)

See comments #225101 (comment 596459737)

See MR gitlab-jh/gitlab!22 (closed)

Risks

Move COM_URL related variables in lib/gitlab.rb to a module and update the codes where using them.

Involved components

app/helpers/version_check_helper.rb

lib/bulk_imports/clients/graphql.rb

lib/gitlab/email/message/in_product_marketing/experience.rb

lib/gitlab/links.rb

lib/gitlab.rb

spec/lib/gitlab/tracking/standard_context_spec.rb

spec/lib/gitlab_spec.rb