make omniauth SAML uid_attribute an ENV variable

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Descriptive summary

As we work on the production SAML configuration, I found that the uid_attribute parameter is a static string in the omniauth config. We should make a SAML_UID_ATTRIBUTE environment variable with the value of "urn:oid:0.9.2342.19200300.100.1.1"

If UO's SAML attributes ever changed, we would need to push a code change through our deployment pipeline and schedule a deployment to production. Making this an environment variable would allow us to just change the value and restart.

https://github.com/OregonDigital/OD2/blob/3a3cbb4b0dc2ab4598d66c11816a1a0c754e9dd6/config/initializers/devise.rb#L257-L263

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while we're there, we can update the default issuer to staging.oregondigital.org instead of the defunct od2-staging.library.oregonstate.edu