Outstanding items for improving the automation

Leftover issues and more from #21.

- [ ] Dry runs/test builds are not currently supported. For that we'll need the script to automatically delete all the histories it created during the run. So the @galaxybot test this and @galaxybot deploy this commands, as well as the automatic run that happens when the PR is opened by an "authorized" user will all deploy.
- ~~Relatedly, run failures require deleting all histories created during that run or they will be skipped on the next run.~~
- [ ] We should probably switch from the no-longer-supported ghprb plugin.
- [ ] CVMFS snapshots so that indexers can run back-to-back with fetches
- [ ] Make histories public and generate a slug
- ~~It should be possible to import builds that have completed without deleting the histories and building again.~~
- [ ] The entire process could run as cron jobs/scheduled Jenkins runs instead of being tied to individual PRs
- [ ] PRs adding new genomes have to be run twice (once to run "stage 0" DMs and again to run "stage 1" DMs)
- [ ] Configure MinIO object store backend on usegalaxy.org
- [ ] Running the importer in Docker (and all the double setup that goes along with it) is not really necessary, it's mainly a holdover from usegalaxy-tools so we can mount `$OVERLAYFS_MOUNT` as `/cvmfs/$REPO`. But we can use the namespace techniques used by https://github.com/cvmfs/cvmfsexec to do this without docker, which should be much quicker/simpler.
- [ ] The builder waits for CVMFS client IDC repo revision to sync, but it needs to reload the tables after the sync happens

I commented out a couple of tasks that I expanded upon in #30.

No files, tests, or entry points are named. Choose one unchecked item from the list, first reviewing the related discussion in #21 or #30 and the referenced cvmfsexec project where relevant; define completion as that item working without regressing the existing automation.

Written by the indexing model from the issue text.