HTTPAPI documentation and fini issue

[vEpiphyte]

• Replace httpapi rst with ipynb.
• Add test coverage for addHttpPort on the cell.
• Ensure that cell http servers have .stop() called on them during fini.

[codecov]

Codecov Report

Merging #1211 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1211      +/-   ##
==========================================
+ Coverage   93.53%   93.55%   +0.02%     
==========================================
  Files         124      124              
  Lines       16611    16613       +2     
==========================================
+ Hits        15537    15543       +6     
+ Misses       1074     1070       -4

Impacted Files	Coverage Δ
synapse/lib/cell.py	93.66% <100%> (+1.14%)	⬆️

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