Append number to prevent duplicate question headers

Since we now have question storage, we can look at existing question headers and
append a number if there's a match for a question filed in the same minute.

Also:

• add body as a constructor parameter to QuestionPoster -- the class can't do

much without the body set, and this simplifies the code in that we don't need
to pass $body to various methods.

• adjust some method visibility (addSignature)
• Move permissions checks into a method to try to make submit() a little more readable

Bug: T222125
Change-Id: Ifc6e2d3ab0536420ec3aab162bf8ce9c47207406