MBS-6984: Preserve the status of the 'target' radio button in merges

If the user tries to enter a 'merge' edit and doesn't enter an edit
note, the form cannot be submitted - as intended. However, the new form
loses the state of the 'target' radio button. Instead, we should make
sure to apply the 'checked' attribute were appropriate.
Merge remote-tracking branch 'ocharles/mbs-6984' into beta

* ocharles/mbs-6984:
  MBS-6984: Preserve the status of the 'target' radio button in merges