Email to developer on plan change performed via API

[guicassolato]

What this PR does / why we need it
This PR makes application plan changes performed by the provider through the API to send a notification message to the buyer as well, as it does when performed in the Admin Portal.

Which issue(s) this PR fixes
Closes THREESCALE-923.

[codecov]

Codecov Report

Merging #709 into master will decrease coverage by <.01%.
The diff coverage is 91.17%.

@@            Coverage Diff             @@
##           master     #709      +/-   ##
==========================================
- Coverage   92.84%   92.84%   -0.01%     
==========================================
  Files        2387     2387              
  Lines       77516    77523       +7     
==========================================
+ Hits        71967    71973       +6     
- Misses       5549     5550       +1

Impacted Files	Coverage Δ
...ts/cinstances/cinstance_plan_changed_event_test.rb	100% <ø> (ø)	⬆️
app/lib/logic/plan_changes.rb	87.5% <ø> (-0.12%)	⬇️
app/observers/message_observer.rb	100% <100%> (ø)	⬆️
...ications/new_notification_system_migration_test.rb	98.41% <100%> (+0.07%)	⬆️
...notifications/new_notification_system_migration.rb	100% <100%> (ø)	⬆️
...n/admin/api/buyers_applications_controller_test.rb	100% <100%> (ø)	⬆️
test/unit/observers/cinstance_observer_test.rb	100% <100%> (ø)	⬆️
test/unit/observers/message_observer_test.rb	100% <100%> (ø)	⬆️
app/mailers/mail_preview.rb	34.88% <33.33%> (+1.14%)	⬆️
test/workers/create_service_token_worker_test.rb	100% <0%> (ø)	⬆️
... and 3 more

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[Martouta]

I like this new version 👍 Good job! 🥇

[hallelujah]

So this class is only for Provider

Provider users can choose which notification they get in their notification preferences
Developers do not really have this option.

@macejmic can we have a way to use subscribe_for_notification in another class?

[macejmic]

not really, we would need to create the same logic for developers.

[Martouta]

LGTM 👍 And I appreciate the tests to understand a bit of what's going on 😄 but I definitely don't know enough about this notifications/emails logic so it would be good if @hallelujah and @macejmic can review too 😄

[hallelujah]

Might be good to use asynchronous mailing.
But I think it is OK for now as it was like that in the controller anyway.

[Martouta]

LGTM 👍