Expose issue stopwatch toggling via API

[adelowo]

.. #4327 automatically stops the stopwatch but this PR will be useful in case there
is a need to programmatically toggle the stopwatch.

Fixes #5942

[codecov-io]

Codecov Report

Merging #5970 into master will decrease coverage by 0.09%.
The diff coverage is 3.27%.

@@            Coverage Diff            @@
##           master    #5970     +/-   ##
=========================================
- Coverage    38.8%   38.71%   -0.1%     
=========================================
  Files         330      330             
  Lines       48731    48853    +122     
=========================================
+ Hits        18911    18912      +1     
- Misses      27081    27201    +120     
- Partials     2739     2740      +1

Impacted Files	Coverage Δ
routers/api/v1/repo/issue.go	23.46% <0%> (-7.81%)	⬇️
routers/api/v1/api.go	73.74% <100%> (+0.2%)	⬆️
modules/process/manager.go	76.81% <0%> (-4.35%)	⬇️

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[lafriks]

I don't actually like toggle method as you are kind of guessing state... I would prefer if API had start & stop methods

[adelowo]

Done @lafriks.