Add profile spam reporting

[brtdwchtr]

Summary

• add an authenticated profile-report endpoint for user profiles
• add a report form with spam, abuse, fraud, and other reasons plus optional comments
• send reports to support@bountysource.com from/reply-to the reporting user's email

Fixes #1148.

Verification

• git diff --check
• ruby -c app/controllers/api/v1/people_controller.rb
• ruby -c app/mailers/mailer.rb
• ruby -c config/routes.rb
• ruby -c spec/controllers/api/v1/people_controller_spec.rb
• ruby -c spec/mailers/mailer_spec.rb
• node --check app/assets/javascripts/app/people/activity.js

Targeted RSpec was not run locally because the repo pins Ruby 2.7.1/Bundler 2.1.4 and this machine only has system Ruby 2.6.10 without the project gem bundle installed.

[CLAassistant]

All committers have signed the CLA.