Interpolate star URL into the plaintext mail

Prior to this commit, the plaintext star mail had the string
'star_url' in it, not the actual evaluated URL interpolated into the
mail.  This commit fixes that issue.

Change-Id: I1b4657bdcaf5fe07e71d3594a46647bdbfb832c8
Reviewed-on: https://gerrit.causes.com/22816
Reviewed-by: Aiden Scandella <aiden@causes.com>
Tested-by: Elliot Block <elliot.block@causes.com>