Make PHONE_NUMBER_COUNTRY configurable at runtime

[jlegrone]

Right now PHONE_NUMBER_COUNTRY must be provided as an environment variable at build time. This may be better suited as a per-campaign/org/instance setting that can be configured at runtime, and would allow building deployment artifacts that work for multiple countries.

Related commit: MoveOnOrg@33e2edc

[ibrand]

@matteosb @schuyler1d we've been chatting a lot about campaign vs org level settings. My feeling is that this would be a reasonable org-level setting, but I'm not sure if it should be per-campaign configurable