Add character limit on 'UpdateElectorate'.

[annatorok]

Context:
Ledger device is not at all good at displaying large amounts of text.

The up to 5000 character description on governance proposals is fine for browsers, but not for the Ledger.

The Ledger is only required to sign the UpdateElectorate message. So, all governance proposals except UpdateElectorate can keep the 5k limit but a 120 character limit has to be imposed on UpdateElectorate. We should impose that limit in the governance web app.