Rename "Blocked in" to "Not distributed in"

We don't have means right now to test whether bridges are actually blocked in, say, Russia right now or not, yet we claim that bridges are blocked in Russia when the assignments file contains blocklist=ru. However, that's only an indicator that the bridges in question are not getting distributed in Russia at the moment (for whatever reason).

Thus, to be less confusing we should rename our "Blocked in" part to "Not distributed in". Note: the tooltip is pretty accurate, though.

(This got brought up at the relay operator meet-up 2024-03-02).