Filter flash to only display info, success and error messages.

Avoid displaying [flash commands](https://github.com/moneyadviceservice/public_website/blob/master/app/controllers/commands/command_helper.rb) set by the public website.
Merge branch 'bug/flash-commands'
Use public website database in development env
Share sessions with public_website
Add Authentication concern
Render login/logout links in site header
Add "register" and "my account" links where appropriate
Add content to EN.yml and fake content to CY.yml
Adjust layout of elements in the header
Change the alignment of the elements in the footer
Add partials for styling the my-account component
Add welsh content to the YML file
Adjust EN and CY content after rebase
Refactor footer sign in and register links
Refactor header sign in and register links
Rename my-account class to authentication
Rename partial and remove old files.
Fix tests to point to the right partial
Fix spacing around the langauge links on tablet
Remove whitespace for Tyler Durden @stevenwilkin