[Feature flag] Enable option to display one file at a time in MRs

What

Remove the :view_diffs_file_by_file feature flag ...

Owners

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

Expectations

### What are we expecting to happen?

Option shows up in the Preferences screen of the user settings.

What might happen if this goes wrong?

Problems limited only to users who CHOOSE to enable this. It's opt-in.

What can we monitor to detect problems with this?

Any reports related to MR bugs need to be checked whether they have this mode ON or OFF.

Beta groups/projects

Since this is user-setting we can't scope it to projects or groups.

Straight to globally enabled, given the safety of the opt-in nature.

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 (more guidance when this is necessary in the dev docs) 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