Prevent duplicate metaAliases in SAML2 entities

The Federation console allows for entities to be created with the same metaAlias. Later, during federation when an entity is retrieved by its metaAlias the search may return a different entity with the same metaAlias.
The console should show an error message when the metaAlias entered matches an existing one, as it does when the 'Entity Identifier' already exists.

To prove that no metaAlias validation is done:
1. Using the 'new' button under 'Entity Providers' in the Federation tab. Add two SAMLv2 IDP entities, entering the same value for the metaAlias field. Both may be created with no validation