#2814 Fix activity:sendEmail to use renderTemplate

Note that this does create a situation where more queries could occur but
1) this is only called in core from the email task - which is limited to 50 contacts so it
is low-volume
2) this is really the start of a cleanup - there are other places where
queries can be removed when we look at the total flow but I feel we need to get
some simplification happening first & hence have picked on this focus
as the first step (ie removing the calls to replaceContactTokens)
3) this seems to be called in extensions to some extent - and it should still work
- there might be more queries but I think the 'support contract' for calling an
unsupported function does not include it working in ways not required by
core code