Mailgun: send e-mail via API

[php-coder]

Switching to their API give us some benefits:

• easier to test (when we have WireMock (Use WireMock for mocking external services #934), we'll be able to mock API)
• easier to consume from different tests and different technologies/languages (at this moment, mail server is available only during execution of the one particular test also we can't use it from Robot Framework because in-memory mail server is available only inside JVM)

See https://documentation.mailgun.com/en/latest/user_manual.html#sending-via-api

Plan:

• prepare curl command for sending e-mail
• test curl command for sending e-mail
• add a curl command to the class comment
• rewrite the strategy to send via HTTP API
• introduce mailgun.endpoint, and mailgun.password properties (in all files + ansible)
• test that sending works
• think about how to turn on a new strategy? OLD or NEW only vs NEW and OLD as a fallback
• think about migration

[php-coder]

#530 is blocked by this issue because WhenAnonymousUserRegisterAccount.successfulMessageShouldBeShownAfterRegistration and emailWithActivationKeyShouldBeSentAfterRegistration are using SMTP that is hard to impossible to implement with Robot Framework.

[php-coder]

This issue is semi-blocked by #1020 because we need to test that the new API works on production.