Change domain name

`fellowship.azavea.com` is misleading because we have two fellowship programs.

### Goals

- Make association between Azavea and fellowship clear
- Enhance branding of company and fellowship program
- Eliminate potential for confusion

### Options

- Nested domain
- azavea.com/fellowships/open-source
- creates option to design landing page for fellowship programs
- provides contingency in the case that additional programs are added in the future
- Subdomains
- osfellowship.azavea.com
- opensourcefellowship.azavea.com

@lossyrob @rcheetham @hectcastro @designmatty:

Note that there is a related issue in the Azavea Open Source Fellowship website repo:
https://github.com/azavea/summer-of-maps-website/issues/43

Can you please comment your thoughts below? Let's address this potential branding issue before it becomes a longterm problem.