Add default_displayname_template

[Martin-Lndbl]

I am having trouble with the displayname_template config option. As multiple people use my bridge I have to rely on .PushName as default name (which is unreliable for many users). I currently manually change the the name of these contacts (which is later overwritten by the bot). What I'm proposing is a default_displayname_template option that only applies the template on creation of a new account, without syncing it periodically.

• In large groups some people still have a meaningful name
• I can freely modify display names in a persistent manner

I believe multiple people have this issue and this seems like a relatively easy way to fix it.

[tduboys]

That's a subject I'm also concerned

but I dont know if the fix must come from the bridge bot or from the Matrix protocol, with feature like matrix-org/matrix-spec#188
I think it's better to use the Matrix specs but it may be more complicated to implement in all the stack (specs + server code + client code).