[MIG] Migrate currency_rate_update to V8.

@nbessi 
I still have to review the security files, waiting for your opinion what needs to change to comply with the OCA rules (being my first big contribution).
This sentence needs to be improved:
"When this option is unchecked it will force users to set a distinct currency on each company."
or something like
"When this option is checked it will allow to use the same curreny on all companies."

But we should avoid double negations.