"Request Re-execution" button

[u8sand]

Because output is cached and because we live in an imperfect world -- it's possible that some external API was unavailable coincidentally during an execution, in this case it may be necessary to re-execute the appyter. Currently I can manually mark an appyter to be re-executed by removing metadata.nbexecute part of the instance's notebook, but it may make sense to allow users to request a re-execution to mitigate permanently broken appyter instances.

Because some appyters can simply be flawed -- we can't just assume errors mean we should re-execute; in any case, the current recommended way to deal with these situations is to submit an issue and let us investigate manually.