Allow users to re-generate random words before enabling e2e

[sunjam]

This is a feature request for allowing users to re-generate their 12 random words offered by Nextcloud before enabling e2e on the desktop client. This is common practice when generating random passwords in a password manager.

I might be mistakened about this particular issue, but... this would mitigate the security concern of running Nextcloud client unencrypted for years until you one day enabling e2e encryption not knowing that the whole time the same mnemonic has been on display for anyone to copy down #2443, even before it was enabled.

Thanks for considering, and Great Work!