addons-server: Internal Mozilla-signed add-ons are legacy but should not have compatibility restrictions

For the foreseeable future, AMO will be getting legacy submissions that are signed with the internal Mozilla certificate, which gives them the ability to be installed in Firefox 57 and above. We need to make an exception for these add-ons in our compatibility restrictions. Specifically:

• It should be possible to upload them without an admin override.
• They shouldn’t have strict compatibility and should appear as compatible in 57 and above.
• They should show the Compatible with 57+ tag.
• They shouldn’t have lower ranking on search results.

For an example, see Firefox multi-account containers.