Moved the review event of review-bot to only be triggered in approvals.

Because we only update the required reviews when someone approves, this
will stop the bot from immediately requesting a new review when someone
comments or request changes as they should have been already notified in
the first batch.

Added if condition on review-bot's trigger so it does not trigger in
`draft` PRs.

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)