Add Support "Server Timing"

[yurtaev]

Add Server-Timing to response headers. Specification https://www.w3.org/TR/server-timing/

Inspired By https://twitter.com/paul_irish/status/829090506084749312

[codecov]

Codecov Report

Merging #928 into master will increase coverage by 0.36%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #928      +/-   ##
==========================================
+ Coverage   83.46%   83.82%   +0.36%     
==========================================
  Files          28       28              
  Lines        1578     1614      +36     
  Branches      235      239       +4     
==========================================
+ Hits         1317     1353      +36     
  Misses        192      192              
  Partials       69       69

Impacted Files	Coverage Δ
debug_toolbar/middleware.py	89.24% <100%> (+1.74%)	⬆️
debug_toolbar/toolbar.py	81.69% <100%> (+0.26%)	⬆️
debug_toolbar/panels/timer.py	84.61% <100%> (+2%)	⬆️
debug_toolbar/panels/__init__.py	90.56% <100%> (+1.2%)	⬆️
debug_toolbar/panels/cache.py	82.17% <100%> (+0.71%)	⬆️
debug_toolbar/panels/sql/panel.py	64.93% <100%> (+1.17%)	⬆️

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[auvipy]

that should be a good inclusion

[auvipy]

try to fetch and resolve conflicts

[camilonova]

@yurtaev Thank you.