Change "Supporters" to "Contributors" on website frontend

[andrew-boyd]

The language in sponsorship soliciting has changed, we should make sure the website reflects it.

[viertaxa]

@andrew-boyd do we want to refactor the the language in the code, too, or just update the header to reflect the new nomenclature?

[dylansheffer]

I think it would be best to go ahead and refactor the code as well as the header just so it's more clear. It would be best if we merge in #25 before the refactor though to prevent conflicts