Ref T9123. To run upstream builds in Harbormaster/Drydock, we need to be able to check out libphutil, arcanist and phabricator next to one another.
This adds an "Also Clone: ..." field to Harbormaster working copy build steps so I can type all three repos into it and get a proper clone with everything we need.
This is somewhat upstream-centric and a bit narrow, but I don't think it's totally unreasonable, and most of the underlying stuff is relatively general.
This adds some more typechecking and improves data/type handling for custom fields, too. In particular, it prevents users from entering an invalid/restricted value in a field (for example, you can't "Also Clone" a repository you don't have permission to see).
