Enable MR :diffs_batch_load feature flag by default

What

This is a reminder to change the feature flag diffs_batch_load to enabled by default. Let's not remove the feature flag yet as it's changing such a core feature of GitLab.

Owners

• Team: Create::Source Code
• Most appropriate slack channel to reach out to: #g_create_source-code
• Best individual to reach out to: @iamphill @kerrizor @andr3 @oswaldo @m_gill

Expectations

### What are we expecting to happen?

What might happen if this goes wrong?

What can we monitor to detect problems with this?

Beta groups/projects

If applicable, any groups/projects that are happy to have this feature turned on early. Some organizations may wish to test big changes they are interested in with a small subset of users ahead of time for example.

• gitlab-org/gitlab project
• jboyson/my-awesome-test-project project

Roll Out Steps

• Enable on staging
• Test on staging
• Ensure that documentation has been updated
• Enable on GitLab.com for individual groups/projects listed above and verify behaviour
• Coordinate a time to enable the flag with #production and #g_delivery on slack.
• Announce on the issue an estimated time this will be enabled on GitLab.com
• Enable on GitLab.com by running chatops command in #production
• Cross post chatops slack command to #support_gitlab-com and in your team channel
• Announce on the issue that the flag has been enabled
• Remove feature flag and add changelog entry
• After the flag removal is deployed, clean up the feature flag by running chatops command in #production channel

/label feature flag