Enable linking to players with multiple identities

[sussexrick]

When on the page to link one player identity to another, we do not offer to link to identities which belong to players with other identities because the UI cannot show that relationship, and it would be counter-intuitive to select one identity and be presented with three extra ones. Update the autocomplete UI to display the linked identities and then enable them to be linked.

• Write an integration test to show that ReadPlayerIdentities will populate the Player.PlayerIdentities collection of each result. This should fail where the PlayerQuery.Query property is used. Make it pass.
• Write tests and a short method to automate generating the "linked to" text, using the logic currently in EditPlayersForTeam.cshtml
• Update PlayersApiController.AutoComplete to be able to return results with the "linked to" text included.
• Update PlayerRespository and its tests to enable these identities to be linked. Update Players.md to say that it's implemented.
• When you select one linked identity it is added to the table (eg Jane Smith aka J Smith). At this point the other linked identities (eg J Smith aka Jane Smith) must be excluded from the next autocomplete dropdown.
• Update Players.md.