Introduce capybara and first feature spec (#1801)

This commit introduces Capybara and the first feature spec.

I focused on coverage for log in for the first feature spec because that would
have prevented 624a9a7136 causing #1236.

Gemfile.lock

@@ -99,6 +99,13 @@ GEM
       sshkit (~> 1.3)
     capistrano-yarn (2.0.2)
       capistrano (~> 3.0)
+    capybara (2.13.0)
+      addressable
+      mime-types (>= 1.16)
+      nokogiri (>= 1.3.3)
+      rack (>= 1.0.0)
+      rack-test (>= 0.5.4)
+      xpath (~> 2.0)
     chunky_png (1.3.8)
     climate_control (0.1.0)
     cocaine (0.5.8)
@@ -446,6 +453,8 @@ GEM
     websocket-driver (0.6.5)
       websocket-extensions (>= 0.1.0)
     websocket-extensions (0.1.2)
+    xpath (2.0.0)
+      nokogiri (~> 1.3)
 
 PLATFORMS
   ruby
@@ -465,6 +474,7 @@ DEPENDENCIES
   capistrano-rails
   capistrano-rbenv
   capistrano-yarn
+  capybara
   coffee-rails (~> 4.1.0)
   devise
   devise-two-factor