Handle cover block on mobile - support or prevent/warn against usage

[bregman90]

We do not currently support the standard cover block well on mobile. Using the cover block introduces horizontal space that makes the page scroll on mobile instead of being vertical scroll only.

From the help desk at WordPress.com: "Yes, the cover image shouldn't add horizontal scrolling at all. :^) It's because the theme doesn't let the cover block break out of the flow and instead adds this extra padding to the left: https://d.pr/i/uGwHRR"

We should either:

1. Allow cover block to function as expected
2. Prevent rendering cover block on mobile
3. Not allow cover block to be added at all in the editor

Below is an image that show how we can scroll the page on mobile (https://planyourvotesa.org/results/):