[Qt] remove space from translation of client bitness

- its rather easy to leave out the space on Transifex, so remove it from
  the string
Make GetAvailableCredit run GetHash() only once per transaction.

This makes the first getbalance/getinfo 63x faster on my wallet.
Merge pull request #3922

c17f0a5 [Qt] remove space from translation of client bitness (Philip Kaufmann)
Merge pull request #3973

a60ab0b Make GetAvailableCredit run GetHash() only once per transaction. (Gregory Maxwell)
Replaced hard-coded value with nHighTransactionFeeWarning as per discussion in #3969.