Make clearer token is required for Micropub

[dshanske]

Suggestions include...generating a link to the appropriate Bridgy page next to the token box. Better error checking if the token is empty, and the fact that if you are triggering via Micropub it still lets you, but if you trigger via the admin it doesn't.

[dshanske]

Also, token verification is possible by querying the Micropub endpoint with the token and checking for a 200 vs 400 series response code.