refactor: rename session server instances in keyshare server to irmaserv for consistency with `irma server`

For consistency with the irma server and myirma server and for a potential future
redis implementation.

feat: prevent large HTTP POST body and slow trickle requests DOS vectors in keyshareserer and myirmaserver

Additionally, email templates may now only be given using files instead
of as template.Template instances directly.

This also solves a previous inconsistency where the server would write failed
pin checks to the user log in case of an ordinary pin check, but not on pin
changing.