Check for push permissions before running a benchmark

[joao-paulo-parity]

It's somewhat pointless to run a weights benchmark if the bot is not able to push the generated weights. Some check should be implemented to avoid running the benchmark in that case and, optionally, provide a way of forcing the run through some extra command.

#54 was proposed but that's supposed to be for recovery in case of unexpected errors, not the default behavior.

Checking for push permissions should be straightforward:

• First, check that the pull request's branch is hosted somewhere the bot can push (e.g. if it's a paritytech repository, then it can).

  • There might be a smarter way of checking for write permissions through the Github API, but maybe it's not worth the extra complication.

• Otherwise, fetch the pull request in the Github API for the field "maintainer_can_modify": true.