Change model name from "RFP" to "Solicitation"

[ajb]

Now that we've added the type field in #10, it'll make more sense if we call our model "Solicitation", which will include RFPs, RFIs, RFQs, etc.

[marks]

+1 As a newcomer to this project, it wasn't clear if only RFPs were of interest or if solicitations/sources sought were also in-scope

[jefflundberg]

I've also heard them called "events", which could be an RFP, RFQ, an auction, etc.