Team owners can permanently merge players

[sussexrick]

Old interface was: if rename matches an existing player name, offer the choice to merge or to pick a new name. Hide the name box until its radio button is chosen.

Merging should be possible without going via the rename route, since that's not obvious, but if both exist they should post to the same place.

UX extends 'Edit player' as amended by #627. It adds a 'Permanently merge players' with 'Pick the name to use for the merged player'.

Player identities can only be merged permanently if they belong to the same team.

Merging code is in player-manager.class.php MergePlayers() method. Merge could work by linking two player identities somehow, which would allow an undo, but that could complicate displaying the correct name.

Must recalculate all the derived statistics data in small batches in a HostedService running outside the main request, to avoid SQL Timeouts. Must therefore display the standard "Statistics may not update immeditately" banner.