Fix regression in edit post and edit topic summary due to OO UI changes
This was a regression in Firefox due to the nodes not being in the DOM.
It's solved by doing the phase that requires being in the DOM outside
of the constructor, when the initializer has put it in the DOM.
This was caused by Ie4f66b15f4f5ec9932d5fd72e12b4453724e3742 .
I believe the specific change was I4109d5ad6089035cf689e11667e6ec2bb007d6ce
which affects how OOjs UI handles text selection ranges.