Use flask.jsonify for json responses

[hyperair]

This ensures that all json responses have the correct mimetype without needing
the extra boilerplate to set the content-type header.

This helps let extensions like JSONViewer do the right thing when examining JSON responses like /stats/requests

[codecov]

Codecov Report

Merging #725 into master will decrease coverage by 3.32%.
The diff coverage is 60%.

@@            Coverage Diff             @@
##           master     #725      +/-   ##
==========================================
- Coverage   69.25%   65.93%   -3.33%     
==========================================
  Files          14       14              
  Lines        1633     1374     -259     
  Branches      292      214      -78     
==========================================
- Hits         1131      906     -225     
+ Misses        445      421      -24     
+ Partials       57       47      -10

Impacted Files	Coverage Δ
locust/web.py	77.31% <60%> (-8.69%)	⬇️
locust/runners.py	51.31% <0%> (-12.61%)	⬇️
locust/core.py	87.82% <0%> (-0.65%)	⬇️

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[heyman]

Looks good, thanks!