There are two reasons this is a good thing:
* first, it means that you don't end up with stuff kicking around your working
copy ending up in the build image by mistake (which can upset the pip
install process)
* second: it means that the docker image cache is more effective, and we can
reuse docker images when iterating on the docker stuff.
since #4298, the optional dependencies are no longer installed with a simple
`pip install .`, which meant that they were not being included in the debian
package.
The easy fix to that is dh_virtualenv --extras, but that needs dh_virtualenv
1.1...
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.
