Add simple validation for parameters when registering new assets.

[octalmage]

This is an idea from Discord around how to prevent polls like this:

https://terra.mirror.finance/gov/poll/127

Could the Mirror frontend query the provided oracle to see if the requested price is there? If not, we should not let the user create a poll.

[josephsavage]

If blocking posts from reaching governance is an issue, we should still query and then include an alert in the UI:

WARNING: This asset does not have a valid oracle!!