Added user certificate REPP endpoint and mailer

[maricavor]

Related to internetee/registrar_center2#93

[viezly[bot]]

This pull request is split into 6 parts for easier review. 👀 Review pull request on Viezly

Changed files are located in these folders:

• /
• app
• app/controllers
• app/lib
• app/models
• config
• lib/serializers/repp
• test/integration
• test/models
• test/system/admin_area