Resolve Devise error messages' deprecation warnings

[rakvium]

DEPRECATION WARNING: [Devise] `DeviseHelper#devise_error_messages!` is deprecated and will be
removed in the next major version.

Devise now uses a partial under "devise/shared/error_messages" to display
error messages by default, and make them easier to customize. Update your
views changing calls from:

    <%= devise_error_messages! %>

to:

    <%= render "devise/shared/error_messages", resource: resource %>

To start customizing how errors are displayed, you can copy the partial
from devise to your `app/views` folder. Alternatively, you can run
`rails g devise:views` which will copy all of them again to your app.
 (called from block in _app_views_devise_registrations_new_html_erb__3296905225930654644_39740 at /home/sloboda/rails_projects/blog/app/views/devise/registrations/new.html.erb:5)
  Rendered devise/shared/_links.erb (Duration: 1.0ms | Allocations: 522)
  Rendered devise/registrations/new.html.erb within layouts/application (Duration: 5.6ms | Allocations: 3543)
  Rendered layouts/_shim.html.erb (Duration: 0.1ms | Allocations: 5)
  Rendered layouts/_header.html.erb (Duration: 3.4ms | Allocations: 489)
  Rendered layouts/_footer.html.erb (Duration: 0.3ms | Allocations: 96)
Completed 200 OK in 115ms (Views: 75.3ms | ActiveRecord: 4.8ms | Allocations: 81758)

[rakvium]

Resolved in #44