Add missing parameter to render_template to grey out UI fields

[max-rocket-internet]

This was missing in #1524, my bad

[codecov]

Codecov Report

Merging #1533 into master will decrease coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1533      +/-   ##
==========================================
- Coverage   81.91%   81.83%   -0.08%     
==========================================
  Files          28       28              
  Lines        2560     2560              
  Branches      388      388              
==========================================
- Hits         2097     2095       -2     
  Misses        365      365              
- Partials       98      100       +2     

Impacted Files	Coverage Δ
locust/web.py	91.81% <ø> (-0.44%)	⬇️
locust/user/task.py	97.29% <0.00%> (-0.55%)	⬇️

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