EditPage: Extract some edit conflict code into EditConflictHelper

EditPage: Extract some edit conflict code into EditConflictHelper

Extensions with custom content models (e.g. ProofreadPage) will extend
EditPage to provide a customized editing experience. However when
another extension like TwoColConflict wants to change just how edit
conflicts look, this causes problems since only one class can take the
place of EditPage.

So instead, split most of the frontend code for edit conflicts into
TextConflictHelper, and call it from edit page. Extensions can override
with the instance created by calling
EditPage::setEditConflictHelperFactory().

And to make that split possible also move EditPage::addNewLineAtEnd()
and ::buildTextboxAttribs() into a separate TextboxBuilder class that
both EditPage and TextConflictHelper can use.

Bug: T176393
Change-Id: Ie415edd84329c02d5762477f8a171fced85b01a2