Purpose of this commit is to clean some bits of code, notably calls to
message_post/log as well as notification methods. It will ease performance
improvement work.

Small optimization: account: read content after extension check

Parameter cleaning

 * use message log with kwargs instead of args;
 * remove message post after hook useless parameters;
 * remove _notify_email_recipients useless message parameter;
 * remove message_notify useless send_after_commit parameters;
 * remove message post params matching default values;

Other improvements

 * remove message_post commands support for partners and channels;
 * only calls message_post with ids list for channels and partners. We
   don't support mix of ids and command anymore to simplify code;
 * remove support of private discussion in mail.thread adding partners
   as recipients, as there is no use anymore;

Related to task 1943901
Linked to PR #32404