Move email address resolution into separate method.
Move email address resolution into separate method.

This is work to support GH-2486, which will require rendering one
message per user instead of the current approach which renders the
content once and generates multiple messages from it.

alexm92/sentry,JamesMura/sentry,gencer/sentry,jean/sentry,zenefits/sentry,jean/sentry,mvaled/sentry,gencer/sentry,fotinakis/sentry,zenefits/sentry,looker/sentry,ifduyue/sentry,jean/sentry,looker/sentry,fotinakis/sentry,mvaled/sentry,JackDanger/sentry,BuildingLink/sentry,BuildingLink/sentry,mitsuhiko/sentry,beeftornado/sentry,fotinakis/sentry,BuildingLink/sentry,JamesMura/sentry,JamesMura/sentry,ifduyue/sentry,looker/sentry,mvaled/sentry,beeftornado/sentry,BuildingLink/sentry,JamesMura/sentry,beeftornado/sentry,JamesMura/sentry,gencer/sentry,jean/sentry,zenefits/sentry,ifduyue/sentry,mitsuhiko/sentry,mvaled/sentry,looker/sentry,gencer/sentry,ifduyue/sentry,BuildingLink/sentry,alexm92/sentry,alexm92/sentry,mvaled/sentry,JackDanger/sentry,JackDanger/sentry,ifduyue/sentry,zenefits/sentry,zenefits/sentry,jean/sentry,gencer/sentry,looker/sentry,fotinakis/sentry,mvaled/sentry