[land-bot] Split land bot logic into distinct scripts

Summary:
This provides a much clearer division of responsibilities.
In so doing, land-patch.sh can be leveraged for both human-authored
revisions and automated commits. apply-revision.sh contains the
revision-specific pre-land logic that the land-bot runs as part of the
arc land-via-bot workflow.

Coincidentally, these pieces are both much easier to test by simply running
them locally.

Depends on D7455

Test Plan:
Try applying some revision:

CONDUIT_TOKEN=<token> ./apply-revision.sh Dxxxx

This effectively only runs smoke tests at this time, but testing land-patch.sh
will become more relevant as autogen gets added:

git checkout master
git checkout <this-patch> land-patch.sh
./land-patch.sh --dry-run

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7467