UI Add /manage_users admin page with a list of users

[rth]

This adds /manage_users page accessible to admins, which allows to search within the user table without having to manually do DB queries.

[codecov]

Codecov Report

Merging #538 (a7f9eb7) into master (a729de1) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #538      +/-   ##
==========================================
+ Coverage   93.47%   93.48%   +0.01%     
==========================================
  Files         103      103              
  Lines        8867     8882      +15     
==========================================
+ Hits         8288     8303      +15     
  Misses        579      579              

Impacted Files	Coverage Δ
ramp-frontend/ramp_frontend/tests/test_admin.py	100.00% <100.00%> (ø)
ramp-frontend/ramp_frontend/views/admin.py	85.97% <100.00%> (+0.62%)	⬆️

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[kegl]

Thanks @rth , the code LGTM. This is a very useful feature on xianti, I think it will also be for ramp.studio. @agramfort ?

[frcaud]

LGTM provided that CI issue is resolved. Thanks @rth.

[frcaud]

Thanks @rth