🧱 Add session history to flowstarts

[rowanseymour]

No description provided.

[codecov]

Codecov Report

Merging #2881 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #2881   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          371       372    +1     
  Lines        25255     25262    +7     
=========================================
+ Hits         25255     25262    +7     

Impacted Files	Coverage Δ
...flows/migrations/0236_flowstart_session_history.py	100.00% <100.00%> (ø)
temba/flows/models.py	100.00% <100.00%> (ø)

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