As a Schema editor I want to be able to merge two Schemas in order to remove duplicates/redundant ones without losing tracability.

Problem:
Entity Schemas currently can not be merged leading to unwanted duplicates staying around.

Acceptance criteria:

• Two Schemas can be merged via an API and special page equivalent to Special:MergeItems
• The merge source is turned into a redirect to the merge target.
• The edit summary of the merge source shows "Redirected to E___" for the edit turning it into a redirect. If needed there is an additional edit before that clears the Schema with the edit summary "Cleared an Entity Schema".
• The Schema Text of the merge source is discarded. Only the one in the merge target is kept.
• Labels, descriptions and aliases from the merge source are added to the merge target if they don't exist there. The edit summary is "Merged Entity Schema from E___". If they exist in the merge target then the ones from the merge source are discarded.