Feature/update confirmation email information (#145)

* Add `invited_by` foreign key to User

Allows for a User to be tied to the user who invited them

* Include `current_user` in new agent initialization parameters

* Add `shoulda-matchers` for testing associations

* Add Inviter information and associated account to welcome email

* Only show inviter info if applicable

* Update conversation spec for FFaker compatibility

Gemfile.lock

@@ -197,6 +197,7 @@ GEM
       railties (>= 4.2.0)
     faraday (0.16.2)
       multipart-post (>= 1.2, < 3)
+    ffaker (2.13.0)
     ffi (1.11.1)
     figaro (1.1.1)
       thor (~> 0.14)
@@ -395,6 +396,8 @@ GEM
       activesupport (>= 4)
     sentry-raven (2.11.3)
       faraday (>= 0.7.6, < 1.0)
+    shoulda-matchers (4.1.2)
+      activesupport (>= 4.2.0)
     sidekiq (6.0.1)
       connection_pool (>= 2.2.2)
       rack (>= 2.0.0)
@@ -475,6 +478,7 @@ DEPENDENCIES
   devise_token_auth!
   facebook-messenger (~> 0.11.1)
   factory_bot_rails
+  ffaker
   figaro
   foreman
   hashie
@@ -505,6 +509,7 @@ DEPENDENCIES
   sass-rails (~> 5.0)
   seed_dump
   sentry-raven
+  shoulda-matchers
   sidekiq
   spring
   spring-watcher-listen