upgraded review bot to v2.1.0 (#1908)

Upgraded to version 2.1.0 which has paritytech/review-bot#94, a change
in the logic of the action to overcome some problems with permissions
coming from PRs from forks

For this, we needed to divide the actions into two files:
- A first action that triggers on PRs and reviews and uploads the PR
number.
- A second action which is triggered under the completion of the first
one and runs as the action normally runs (but won't have any problems
regarding permissions because it is triggered from the master branch)