Refs T15094

This is a merge of the Phabricator master branch into Phorge's master branch.

Created this by

1. Fetching/pulling latest changes from phorge
2. Adding new remote from the phacility/arcanist repository on GitHub
3. Fetched the master branch from this new remote
4. Checked out origin/master (phorge)
5. Ran git merge phab/master to merge the phabricator master branch changes into phorge's master
6. There were conflicts in these files:

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	both modified:   resources/celerity/map.php
	deleted by them: scripts/install/install_rhel-derivs.sh
	deleted by them: scripts/install/install_ubuntu.sh
	both modified:   src/docs/user/installation_guide.diviner

• I re-added the install_*.sh scripts which I believe Phorge is intending to continue supporting while upstream has decided to remove it rather than try to fix/update for branding.
• The installation_guide.diviner document I manually resolved conflicts which was mostly straightforward. I believe Phorge added some extra content (or upstream removed it).
• For celerity/map.php I deleted the file and regenerated it by running:

$ ../arcanist/bin/arc liberate
$ ./bin/celerity map

tbd