BF: Fix bug where local PsychoJS server kept running after quitting

[TEParsons]

We were killing it on a re-run of experiments, but not on the app closing. This fix also moves the starting and killing server code to their own functions so we only have to change it in one place, and keeps track of the port so that we can print when the server is closed (useful for debugging)

[codecov]

Codecov Report

Merging #6323 (bcd9967) into dev (b8c07a3) will decrease coverage by 0.01%.
Report is 36 commits behind head on dev.
The diff coverage is 20.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #6323      +/-   ##
==========================================
- Coverage   51.87%   51.87%   -0.01%     
==========================================
  Files         334      335       +1     
  Lines       61679    61711      +32     
==========================================
+ Hits        31997    32012      +15     
- Misses      29682    29699      +17     

Components	Coverage Δ
app	∅ <ø> (∅)
boilerplate	∅ <ø> (∅)
library	∅ <ø> (∅)
vm-safe library	∅ <ø> (∅)