move the example email templates into the synapse package so that they can be
used as package data, which should mean that all of the packaging mechanisms
(pip, docker, debian, arch, etc) should now come with the example templates.
In order to grandfather in people who relied on the templates being in the old
place, check for that situation and fall back to using the defaults if the
templates directory does not exist.
The URLs used for notification emails were hardcoded to use either matrix.to
or vector.im; but for self-hosted setups where Riot is also self-hosted it
may be desirable to allow configuring an alternative Riot URL.
Fixes#1809.
Signed-off-by: Adrian Perez de Castro <aperez@igalia.com>
