Add Twilio, RDS terraform provisioning

[tdooner]

This adds documentation for a few more configuration settings that need to be set:

• deploy_base_url (e.g. 'multnomah.clientcomm.org')
• TF_VAR_database_password
• TF_VAR_twilio_auth_sid/ token

But in exchange, we can automatically provision twilio numbers and make sure that their configuration is specified in code. Note that the twilio functionality for terraform is a poorly-maintained 3rd party plugin. The code appears to be of pretty good quality, but you need to build the plugin yourself. (I have a built copy locally that I will attempt to get published.)

Also, RDS. This provisions a simple, single-node database cluster. I think I got the networking right, so as to firewall it from the internet and any other stuff in the VPC that isn't in the web nodes subnet, but I need to do more testing to be sure. I wrote this TODO list in a commit message which I plan to address after I get clientcomm up and running for the first time:

• [ ] Create multiple instances, for reliability purposes
• [ ] Configure backups/snapshots
• [ ] Use a better instance type that db.t2.small (?)
• [ ] Verify security group / firewall connectivity

[codecov-io]

Codecov Report

Merging #353 into master will increase coverage by 0.02%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #353      +/-   ##
==========================================
+ Coverage    63.2%   63.23%   +0.02%     
==========================================
  Files         109      109              
  Lines        4175     4175              
  Branches      456      456              
==========================================
+ Hits         2639     2640       +1     
+ Misses       1536     1535       -1

Impacted Files	Coverage Δ
app/lib/em-notify.js	62.26% <ø> (+1.88%)	:white_check_mark:

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