Add AES passphrase encryption of giver/taker EC/RSA privkeys

* Add AES passphrase entry to prep page, which after key generation
  encrypts the privkeys with the given passphrase.
* Change sign and cashout2 pages to request the passphrase instead of
  pasted privkeys.
* Extend Mw accordingly; adding the two encrypted privkeys (EC and RSA)
  for giver/taker to enter-contract API and then return these in
  subsequent pages so they are available in the pages when submitting
  T2 and T3 signatures.
Merge branch 'gustav/passphrase_encryption'

* gustav/passphrase_encryption:
  Add AES passphrase encryption of giver/taker EC/RSA privkeys