Encourage use of the review function

[Sobsz]

I was thinking about this comment in #2261:

Anything that pushes us toward a "single-gatekeeper" model (which we will de facto have, since no one other than CK has the time or inclination to review vast quantities of sentences)

I was originally going to suggest only adding an option for finding unreviewed sentences, so that it would be easier for people to help with this issue before rolling out the rest of the suggestion. However, I realized that next to no sentences outside of English are reviewed anyway, likely because of the review function still being opt-in (as far as I know). This means that many people who'd otherwise love to contribute to Tatoeba without needing to be creative (this especially applies to monolingual speakers) might never know about reviews existing!

I think that encouraging usage of the review function would go a long way in resolving the issue, whether that's by:

• making the review function be on by default
• adding a more streamlined interface for reviewing (like what's in Common Voice's sentence collector)
• explicitly encouraging people to review N (= 2?) sentences for each one they submit
• some other creative way?