matrix-synapse: Use recommended reverse proxy configuration

From https://github.com/matrix-org/synapse/blob/master/docs/reverse_proxy.rst:

NOTE: Your reverse proxy must not 'canonicalise' or 'normalise' the requested URI in any way (for example, by decoding %xx escapes). Beware that Apache will canonicalise URIs unless you specifify nocanon.

Their recommended config is:

ProxyPass /_matrix http://127.0.0.1:8008/_matrix nocanon
ProxyPassReverse /_matrix http://127.0.0.1:8008/_matrix

However, in our current Apache2 matrix-synapse-plinth.conf, we just use

ProxyPass /_matrix http://localhost:8008/_matrix

I guess we should consider to adopt using nocanon and ProxyPassReverse, unless we have good reasons against them?