Fix and simplify creation of channel logs in IVR handlers

[rowanseymour]

Keep trying and failing to come up with a generic pattern for logging all requests - problem is handler wrapper functions don't mix and different handlers need to resolve different things in order to create logs - e.g. ticketer, channel etc. But this is a start and simplifies the creating channel logs for IVR handlers.

[codecov]

Codecov Report

Merging #349 into master will decrease coverage by 0.04%.
The diff coverage is 39.72%.

@@            Coverage Diff             @@
##           master     #349      +/-   ##
==========================================
- Coverage   54.76%   54.71%   -0.05%     
==========================================
  Files         106      106              
  Lines        8926     8872      -54     
==========================================
- Hits         4888     4854      -34     
+ Misses       3334     3323      -11     
+ Partials      704      695       -9     

Impacted Files	Coverage Δ
web/ivr/ivr.go	45.83% <33.33%> (-4.97%)	⬇️
models/channel_logs.go	54.71% <100.00%> (+5.78%)	⬆️

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[rowanseymour]

This also fixes that currently we don't properly log responses to IVR requests to mailroom as we don't log headers - only the body