Merge #11605: [Wallet] Enable RBF by default in QT

Merge #11605: [Wallet] Enable RBF by default in QT

5cbbbd7 [Wallet] Use RBF by default in QT only (Sjors Provoost)

Pull request description:

  ~If there are no objections, this would supersede #11556.~

  Enabling RBF by default avoids the need to explain all possible use cases of RBF.

  This PR does not change the default RPC wallet behavior, as this could break implementations that depend on it and it's not clear what happens when automated services suddenly switch on RBF on a large scale.

  After trying various approaches, we settled on just having QT ignore `-walletrbf`.

  Send screen:
  <img width="388" alt="send" src="https://user-images.githubusercontent.com/10217/34251097-329c8dee-e63f-11e7-9e14-d7f55d2b52cc.png">

  Confirmation screen by default (with RBF):
  <img width="429" alt="rbf yes" src="https://user-images.githubusercontent.com/10217/32442799-f50d54aa-c2fc-11e7-9392-96339d0f1f74.png">

  Confirmation screen without RBF:
  <img width="431" alt="rf no" src="https://user-images.githubusercontent.com/10217/32442793-ef30bc34-c2fc-11e7-8ca2-e86a97175278.png">

Tree-SHA512: 53efb5d277144478143e69dcae8112c1b9c2beb981fdd0fe778592e5f7d5bf838f73d48052ead874586a75b944e8af469b25e5f376c135cf48cc3598e77f5891