UI doesn't permit slide around "13th circle"

[blahedo]

If the user clicks on a stone in the outermost position (F), it should be able to slide around the invisible 13th circle (i.e. to the F positions on adjacent circles and around until they're blocked). As seen in this screenshot, the user has selected the stone at F4 to move, but the 13th-circle options are not available:

This is in game # 5797941e3a0ac6306cd86c11 .

[blahedo]

Aaand I just confirmed that it's not just the UI; the internal engine is also rejecting these kinds of slides. On this board (game 5799a5313a0ac6306cd86cfe ):

the bot tried the following:

Sliding from F12 to F8
The server has rejected the most recent action.
Sliding from F12 to F9
The server has rejected the most recent action.
Sliding from D3 to E2
Received action:
Player white slides a piece from D3 to E2.

That is, it tried F12->F8, then F12->F9, and only when it tried D3->E2 was the move accepted.

[blahedo]

It looks like the quickest fix to this would be in shared/Validators.js, adding a special case that if both sliding and move are in ring F, that slidable is true. That overpermits (i.e. it doesn't prevent a slide that is blocked by another stone), but that's a separate bug.