[Mainlining] Update account_validity requests

[giomfo]

During the mainlining of synapse, a new optional module has been introduced to handle the account validity.
This involves a new path for the account_validity requests:
/_matrix/client/unstable/account_validity/* is replaced by /_synapse/client/email_account_validity/*
The request format does not change

For the moment the new module has not been deployed

This path is used client side to request a new email or submit a token (send_mail and renew)
I don't think we are using the /renew API

1- Check the occurrences of the AccountValidityAPI use
2- Sync with Ops to set up the account_validity module on DEV platform
3- Update the service to use/test the new path on DEV platform
4- Sync with the Web, Mobile and Ops teams to define a plan to update Tchap clients and the Prod/Preprod infra

[NicolasBuquet]

The only place account_validity API path is used on iOS side is in AccountValidityService class (in Managers) in public method requestRenewalEmail(completion: @escaping (MXResponse<Void>) -> Void) -> MXHTTPOperation?.

This method is only called in AppCoordinator class (in Riot/Modules/Application) to request the sending of a new email with activation link in case the current account is expired.

[NicolasBuquet]

See Android issue : tchapgouv/tchap-android#846

[NicolasBuquet]

Done as in Android.