massive refactoring to switch from camelCase to snake_case:

historically, Libervia (SàT before) was using camelCase as allowed by PEP8 when using a
pre-PEP8 code, to use the same coding style as in Twisted.

However, snake_case is more readable and it's better to follow PEP8 best practices, so it
has been decided to move on full snake_case. Because Libervia has a huge codebase, this
ended with a ugly mix of camelCase and snake_case.

To fix that, this patch does a big refactoring by renaming every function and method
(including bridge) that are not coming from Twisted or Wokkel, to use fully snake_case.

This is a massive change, and may result in some bugs.

sat/plugins/plugin_xep_0391.py

@@ -73,7 +73,7 @@ class JET:
 
     def __init__(self, host):
         log.info(_("XEP-0391 (Pubsub Attachments) plugin initialization"))
-        host.registerNamespace("jet", NS_JET)
+        host.register_namespace("jet", NS_JET)
         self.host = host
         self._o = host.plugins["XEP-0384"]
         self._j = host.plugins["XEP-0166"]
@@ -94,7 +94,7 @@ class JET:
             self._add_encryption_filter
         )
 
-    def getHandler(self, client):
+    def get_handler(self, client):
         return JET_Handler()
 
     async def _on_initiate_elt_build(