Add rack-attack gem

[RamilyaNigmatullina]

Summary

• add rack-attack gem
• add base config

How it works

It protects the app from bad clients. It allows only 300 requests from one IP address per 5 minutes.

Test plan

Send any request that exceeds the limit.

Review notes

While reviewing pull-request (especially when it's your pull-request),
please make sure that:

• you understand what problem is solved by PR and how is it solved
• new tests are in place, no redundant tests
• DB schema changes reflect new migrations
• newly introduced DB fields have indexes and constraints
• there are no missed files (migrations, view templates)
• required ENV variables added and described in .env.example and added to Heroku
• associated Heroku review app works correctly with introduced changes

Deploy notes

None.

References

• Resolves Integrate Rack::Attack to protect app from bad clients #111 Issue