Selfo: pie rule via the Pass button, clearer pass message

[fjvico]

Thanks for merging Selfo and for migrating it to the new i18n structure.
This is a small follow-up, targeting develop:

• Pie rule: right after the first move, the second player can swap sides
  by pressing Pass. The engine handles it (pass is played as pie: every
  piece changes colour and the first player moves again). At any other time
  Pass is rejected.
• The "cannot pass" message now explains that swapping sides is only
  available to the second player right after the first move.
• The custom "Swap colours" button is gone (and with it the
  custom-buttons flag); the generic Pass button does the job.

Testing:

• npm run build passes (including the locale checks; apgames.json
  regenerated with npm run merge-apgames-en).
• All Selfo tests pass, including new ones for the pie turn and for the
  rejected pass. The full suite passes with --timeout 20000; with the
  default 2 s timeout a few unrelated heavy tests (Tumbleweed, Gliss,
  Stibro) time out on my machine.
• I only tried it in the local playground. I don't know how the live
  front exposes a Pass button on the second player's first turn, so
  feedback on the dev server is welcome.

Question: you mentioned the pie flag isn't needed when the engine handles
the swap. I kept it, as in the merged version, because I'm not sure of the
side effects of removing it. Should I drop it in this PR?