Initialize flask_mail when sending mails

Since Mail objects read configuration from application only once, at
initialization time, configuration we got from environment at
altai_api.main() was ignored before this commit. This bug was
introduced at commit:73c69cc and spotted by Alessio Ababilov.

We decided to make an object when we actually need it, which fixes
configuration issue and allows to change configuration on fly. Mail
objects are rather lightweight, so there is nothing bad in creating new
object on every mail.

Change-Id: Ided58a3859fa6289ee35cd4f3b6077b1696424fe