REC.BG.01 Forbid users to change their display name

[Adrien Touzouli]

As username will contain sensitive information (about profession) we don't want users to be able to modify it.
Apparently, there's a similar feature implemented in Tchap.

[Adrien Touzouli]

Hey team! Please add your planning poker estimate with Zenhub @JDixmier @heroinedor

[Adrien Touzouli]

A simple Synapse config : https://element-hq.github.io/synapse/latest/usage/configuration/config_documentation.html#enable_set_displayname

[Adrien Touzouli]

Done : #453