Miscellaneous customization issues

[gsrohde]

• [x] Decide on names for customization templates and check them in (e.g. terraref-application.yml).
• [x] Check in custom images referred to by customization templates.
• [ ] Decide whether to allow "additional styling" customizations for elements other than the sponsors list items.
• [ ] Document: provide a "how-to" guide for customization.
• [ ] Review pull request https://github.com/PecanProject/bety/pull/467.