Update identity provider list

[mogul]

NSF is no longer registered as an IdP, but many other agencies are.

Changes proposed in this pull request:

• Remove NSF
• Add many others

Security Considerations

[Note the any security considerations here, or make note of why there are none]

None, this is just a documentation update; the list of IdPs is already on the login page.

[mogul]

Note that the directions here should also remove NSF, and account (hah!) for the other IdPs in the list somehow. I don't know what restrictions there are/aren't, so I didn't try to do that here.

[markdboyd]

@mogul I'm not sure if we should explicitly list all the integrated providers. The list will always be changing and it seems like it will lead to perpetual documentation rot

[mogul]

Up to you, I was just trying to unrot it.

[markdboyd]

@mogul I'm going to opt to remove the explicit agency references instead. See #2512. Thanks for bringing this to our attention!