FIX bokeh plot in admin dashboard (#461)

[rth]

Backport of #461 to master.

[agramfort]

not clear what this fixes. I see the plot here https://ramp.studio/event_plots/meg_datacamp2020 for example

[rth]

I'm not in that event so I don't see it. But for https://ramp.studio/events/air_passengers_py4ds2020/dashboard_submissions for instance I see,

so no graph, with a Uncaught SyntaxError: expected property name, got '{' syntax error in the JavaScript console. This fixes it.

[rth]

For https://ramp.studio/events/meg_datacamp2020/dashboard_submissions I also see no plots, with the same error both in Chrome and Firefox. So I'm not sure why it's working for you..

[codecov]

Codecov Report

Merging #477 (c716a76) into master (a2f585f) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #477      +/-   ##
==========================================
- Coverage   93.56%   93.54%   -0.02%     
==========================================
  Files          99       99              
  Lines        8496     8496              
==========================================
- Hits         7949     7948       -1     
- Misses        547      548       +1     

Impacted Files	Coverage Δ
ramp-frontend/ramp_frontend/views/visualization.py	7.69% <0.00%> (ø)
ramp-engine/ramp_engine/base.py	92.30% <0.00%> (-1.29%)	⬇️

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[agramfort]

ok I can replicate !

[rth]

Merging with one approval as this should be very low risk and a useful bug fix.