Move post validation logic to syncForEach loop so bot doesn't quit if selected post isn't an image/gif

[RalphORama]

In bot.js, we choose the first post that's found in /hot and don't check the MIME type until after we've quit out of the loop.

Logic flow should instead work like

• syncForEach loop over /hot
  • fetch individual post
  • check content type
    • next('done') if content type is acceptable
    • next() otherwise
• fetch target post
  • filter title
  • download content
  • postMediaChunked
  • post tweet