Bug 1875785: replace target attr in links: wait for the block to load

Whether an external <a> link has target=_blank or not we should open them in external tab/window to differentiate mahara site from external content

check https://bugs.launchpad.net/mahara/+bug/1802205

behatnotneeded

Change-Id: I371a9f63132ae49eedd7543b682fe80602130117
(cherry picked from commit 9f6a3896)