Use response_to_external_id instead of response_to_id

[rowanseymour]

No description provided.

[codecov]

Codecov Report

Merging #389 (42f1110) into main (3447101) will increase coverage by 0.03%.
The diff coverage is 40.00%.

@@            Coverage Diff             @@
##             main     #389      +/-   ##
==========================================
+ Coverage   71.36%   71.40%   +0.03%     
==========================================
  Files          93       93              
  Lines        8257     8252       -5     
==========================================
- Hits         5893     5892       -1     
+ Misses       1762     1758       -4     
  Partials      602      602              

Impacted Files	Coverage Δ
backends/rapidpro/backend.go	61.60% <0.00%> (ø)
backends/rapidpro/msg.go	68.39% <ø> (-0.15%)	⬇️
handlers/test.go	0.00% <0.00%> (ø)
msg.go	33.33% <ø> (ø)
test.go	33.46% <0.00%> (+0.50%)	⬆️
handlers/facebook/facebook.go	82.27% <100.00%> (ø)
handlers/facebookapp/facebookapp.go	80.00% <100.00%> (ø)

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