Sugar Pond CAS -> Intercode OAuth2

[nbudin]

The goal is to switch from using Sugar Pond Accounts for authentication over to using Intercode accounts. There are a few reasons for this:

• Sugar Pond Accounts is CAS-based, whereas Intercode is an OAuth2/OpenID Connect server, which is much better supported in the Ruby ecosystem
• Larp Library is now a NEIL project (actually, it basically has been for most of its life - even though it started as an Alleged Entertainment project, it transferred to NEIL within a few months) and should use a NEIL-controlled authentication server
• Using Intercode for authentication enables some possible future features such as automatically pulling a list of cons where games have run, and/or automatically generating proposals to run Larp Library games at future cons

This is a forward-port of an abandoned feature branch to do this. We've since extracted a gem from that branch called intercode_client that makes the OAuth client stuff reusable, so this new implementation uses that. Otherwise, it's very similar to the older one.