🌐 Add endpoint to change a flow language

[rowanseymour]

Contains #293 which should be merged first

[codecov]

Codecov Report

Merging #292 into master will increase coverage by 0.01%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #292      +/-   ##
==========================================
+ Coverage   52.78%   52.79%   +0.01%     
==========================================
  Files         101      101              
  Lines        8884     8896      +12     
==========================================
+ Hits         4689     4697       +8     
- Misses       3512     3514       +2     
- Partials      683      685       +2     

Impacted Files	Coverage Δ
web/flow/flow.go	69.23% <66.66%> (-0.77%)	⬇️

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