Add SURFconext as authentication option

[JeltevanBoheemen]

See https://wiki.surfnet.nl/display/surfconextdev/Connect+in+5+Steps for implementation details.

Advantages:

• Researchers of other institutions do not need to register for I-Analyzer
• Allow corpus access based on being a member of institution
• We would be conexted 😎

Disadvantages:

• The gain is minimal with current corpora. All corpora that require Solis login are limited to UU staff only, which would not improve when logging in through SURFconext

[lukavdplas]

I think this is a duplicate of #1581 ?

[JeltevanBoheemen]

Yep!