Improve constants in messages.js

[tdooner]

• Don't import twilio, it's unnecessary.
• Use destructuring for credentials for terseness.
• Update the logic for TESTENV to make it an actual constant.
• Update all usages of CCENV to use the credentials-exported one.

[codecov-io]

Codecov Report

Merging #337 into master will decrease coverage by -0.75%.

@@            Coverage Diff             @@
##           master     #337      +/-   ##
==========================================
- Coverage   63.17%   62.43%   -0.75%     
==========================================
  Files         108      108              
  Lines        4166     4161       -5     
  Branches      454      454              
==========================================
- Hits         2632     2598      -34     
- Misses       1534     1563      +29

Impacted Files	Coverage Δ
app/models/messages.js	45.25% <75%> (-0.66%)	:x:
app/controllers/clients.js	26.28% <ø> (-5.72%)	:x:
app/models/clients.js	71.94% <ø> (-5.43%)	:x:
app/models/conversations.js	68.3% <ø> (-2.19%)	:x:
app/lib/em-notify.js	62.26% <ø> (+1.88%)	:white_check_mark:

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