feat(gov): consider disallowing validator vote on text proposal only

[tbruyelle]

#10 removed the ability for validators to vote on any proposal. This can be a problem for param changes and upgrade proposals, because it makes it harder to get a quorum.

Let's consider reinstating validator votes on such proposals.

[tbruyelle]

After discussions during the 02/20 townhall, since validators that aren't aligned with prop 848 will be allowed to participate in govgen, we don't want to give any chance for malicious actions, as can be done with parameter change or upgrade proposals.