Read templating from new templating msg field instead of metadata

[rowanseymour]

Requires nyaruka/mailroom#233

[codecov]

Codecov Report

Attention: Patch coverage is 65.11628% with 15 lines in your changes are missing coverage. Please review.

Project coverage is 74.54%. Comparing base (a081fe6) to head (5123aff).

Files	Patch %	Lines
test/msg.go	75.00%	6 Missing ⚠️
backends/rapidpro/msg.go	16.66%	5 Missing ⚠️
handlers/test.go	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #729      +/-   ##
==========================================
+ Coverage   74.50%   74.54%   +0.04%     
==========================================
  Files         110      110              
  Lines       13307    13263      -44     
==========================================
- Hits         9914     9887      -27     
+ Misses       2674     2664      -10     
+ Partials      719      712       -7     

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