Only add new globus scopes during plugin load

[Xarthisius]

Fixes #534 (for now...). Avoids doubling deriva scope in oauth requests. I'll note that a better solution would be to use set() for scopes in oauth provider, but I don't want to mess with girder proper for now.

How to test?

1. See 500 error when logging in with ORCID #534
2. Alternatively:

   $ docker exec -ti -u root $(docker ps --filter=name=girder -q) gosu girder:girder girder-shell
   > from girder.plugins.oauth.providers.globus import Globus
   > Globus._AUTH_SCOPES
   

   verify that there are no duplicates.

[codecov]

Codecov Report

Merging #535 (78957b3) into master (df40eef) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #535      +/-   ##
==========================================
+ Coverage   92.86%   92.88%   +0.02%     
==========================================
  Files          58       58              
  Lines        4554     4556       +2     
==========================================
+ Hits         4229     4232       +3     
+ Misses        325      324       -1     

Impacted Files	Coverage Δ
server/__init__.py	90.59% <100.00%> (+0.39%)	⬆️
server/lib/deriva/auth.py	41.17% <100.00%> (-6.20%)	⬇️

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[craig-willis]

wfm