Fix service migration(s)

Created by: dblessing

Migration https://github.com/gitlabhq/gitlabhq/blob/master/db/migrate/20140907220153_serialize_service_properties.rb causes problems if a user upgrades directly from pre-7.3 to 7.4. The ultimate reason is that code was changed in Slack service as part of https://github.com/gitlabhq/gitlabhq/commit/20179772140ff0bdc2d61852c0a4f48ee6e4a2d0. When this code change is pulled in prior to the above mentioned migration, the webhook property is not present, which causes validation to fail.

One option is that we could opt to not run validations on the migration.