Team owners can link player identities

[sussexrick]

This refers to linking multiple identities to a single Player, not permanently merging identities like #118.

• UX extends the 'Edit player' page of Rename player identities #626. Now it offers 'Include another player in the statistics for [Jane Smith]' with a related item editor to pick a player from the same team.
• An identity cannot be linked if it is already linked to this or another identity (this might be a longer-term goal but the UI is not feasible for the first iteration).
• [-] If a team owner is also a club owner, allow them to link any player in the club rather than just the single team. Needs a rethink as you can add any team to your club temporarily, so this would be elevation of privilege to linking any player from any team.
• In "players linked to my account" show who created the link, "me" or "team owner". Removing an identity linked by a team owner should be unavailable.
• Similarly team owners cannot break a link created by a member.
• This will require a new field on PlayerIdentity.
• Like linking a player to your member account, updates to derived statistics must be done in small batches by a HostedService running in the background. The existing HostedService for linking a player to a member account may already do what's required for this.
• When linking or unlinking an identity, determine whether it's being done by a team owner or Stoolball England administrator and record that
• Do not display the context identity in the table of linked identities
• Update the content page describing the best option for tidying up player lists
• Review scenarios in my documentation against integration tests
• Review scenarios in my documentation against UI behaviour