Can't access to /_synapse

[PxBCPG]

Describe the bug

I can't access to the /_synapse URL. In consequences, I can't access to the synapse admin API

Steps to reproduce

Try to connect to yourhomeserver/_synapse, and you will be redirected to a yunohost page

Expected behavior

This URL is not linked to synapse, or appeared to be deviated by Yunohost

Solution proposed

As we don't often use the synapse API, it might be a good idea to set it as a permission, for example, or an option that we can tick when we need it and that we can close when we don't.

[Josue-T]

Closing as it's a dupplicate of #305