Read Only Legacy Feature Flags

Make legacy feature flags read only in the UI.

Continue to allow them to be changed via the API.

Legacy flags can still be deleted.

This issues is only to write the code. The feature should not be enabled until %13.4.

There is an issue to enable the feature flag and rollout the read only flags here: #220227 (closed)

Changes to be made

On the edit page:

• Remove the Save Changes button.
• Change the warning message at the top to read, "GitLab is moving to a new way of managing feature flags. This feature flag is read-only, and it will be removed in 14.0. Please create a new feature flag."
• On the backend, refuse PUT requests to update legacy flags (the request made when clicking the Save Changes button).

On the index page:

• Disable the Status switch for legacy flags.
• Change the tooltip for legacy flags to read "Flag is read-only".
• On the backend, refuse PUT requests made when trying to toggle the flag with the "Status" switch.