Add the ability to disable the admin UI

Here's a basic patch that works. Allows you to define a variable of the form:

metatag_enable_ui_{$entity_type}__{$bundle}
metatag_enable_ui_{$entity_type}

To disable the admin form for a particular entity type and/or bundle, just set this var to FALSE, it defaults to TRUE. eg.

// Disable metatags UI for carousel nodes.
$conf['metatag_enable_ui_node__carousel'] = FALSE;

// Disable metatags UI for file_entity.
$conf['metatag_enable_ui_file'] = FALSE;

@DamienMeKenna if you disable the field for the bundle on the settings page, it will disable the metatag config for that bundle as well as the admin form right? In our case, we wanted to disable just the admin form to save bloat on the form and to avoid a scenario where values were saved inadvertently for an entity, which then overrides the default metatag settings.

Here is an updated patch against 7.x-1.7, which we are currently testing.

Oops, didn't make that patch relative. Updated.