Move toolbar.timer.js → timer.js to match naming convention

[jdufresne]

All other panel specific JavaScript files use the same name as their
panel. So too should the timer panel.

[codecov]

Codecov Report

Merging #1379 into master will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master    #1379   +/-   ##
=======================================
  Coverage   87.83%   87.83%           
=======================================
  Files          29       29           
  Lines        1578     1578           
  Branches      222      222           
=======================================
  Hits         1386     1386           
  Misses        141      141           
  Partials       51       51           

Impacted Files	Coverage Δ
debug_toolbar/panels/timer.py	79.66% <0.00%> (ø)

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[matthiask]

Thanks!