interpolate account domain in announcements

When displaying the announcement, substitute
occurrences of the string "{{ACCOUNT_DOMAIN}}" (also
the escaped string "%7B%7BACCOUNT_DOMAIN%7D%7D") 
with the user's account domain.

Solution for CNVS-8318 (that is to say, we can
use account domain as a GET parameter which we can use to
differentiate amongst the results we get from
fluidsurveys)

Test plan:
- Create an announcement with the string #{ACCOUNT
_DOMAIN} in the message
- Check the display of that announcement to confirm
the string has been substituted with the user's
root account domain

Change-Id: I338dab609e206c3b47d4c009dda876616959da13
Reviewed-on: https://gerrit.instructure.com/27626
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Anthus Williams <awilliams@instructure.com>
Product-Review: Anthus Williams <awilliams@instructure.com>
QA-Review: Anthus Williams <awilliams@instructure.com>