Replace 'perspectives' config block with 'trusted_key_servers' in docker homeserver.yaml template (#9157)

Signed-off-by: Mateusz Przybyłowicz <uamfhq@gmail.com>

Removes any `SMTP_*` docker container environment variables from having any effect on the default config.

Fixes https://github.com/matrix-org/synapse/issues/5430

... to help people escape env var hell

* no_tls is now redundant (#4613)
* we don't need a dummy cert any more (#4618)

Also:

* Fix wrapping in docker readme
* Clean up some docs on the docker image
* a workaround for #4554

* remove dh_params and set better cipher string

These aren't used, because we have a `log_config` setting.

Signed-off-by: Cédric Laudrel <dek@iono.me>

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.

This addresses #3224