Add rate limits for logins and sign-ups by IP (5 in 5 minutes) (#2079)

* Add rate limits for logins and sign-ups by IP (5 in 5 minutes)
Should be enough for normal attempts

* Add rate limit for forgotten password form as well

config/initializers/rack_attack.rb

@@ -0,0 +1,36 @@
+# frozen_string_literal: true
+
+class Rack::Attack
+  # Rate limits for the API
+  throttle('api', limit: 300, period: 5.minutes) do |req|
+    req.ip if req.path =~ /\A\/api\/v/
+  end
+
+  # Rate limit logins
+  throttle('login', limit: 5, period: 5.minutes) do |req|
+    req.ip if req.path == '/auth/sign_in' && req.post?
+  end
+
+  # Rate limit sign-ups
+  throttle('register', limit: 5, period: 5.minutes) do |req|
+    req.ip if req.path == '/auth' && req.post?
+  end
+
+  # Rate limit forgotten passwords
+  throttle('reminder', limit: 5, period: 5.minutes) do |req|
+    req.ip if req.path == '/auth/password' && req.post?
+  end
+
+  self.throttled_response = lambda do |env|
+    now        = Time.now.utc
+    match_data = env['rack.attack.match_data']
+
+    headers = {
+      'X-RateLimit-Limit'     => match_data[:limit].to_s,
+      'X-RateLimit-Remaining' => '0',
+      'X-RateLimit-Reset'     => (now + (match_data[:period] - now.to_i % match_data[:period])).iso8601(6),
+    }
+
+    [429, headers, [{ error: 'Throttled' }.to_json]]
+  end
+end