add Settings model

[#69344414]
add Settings controller and routes

[#69344414]
add Settings edit view and form

[#69344414]
add link to Settings for admins

[#69344414]
add Settings.notifications_email to note_mailer

[#69344414]
spec for Settings.instace#notifications_email

[#69344414]
add slug to Backbone view for new project

[#73458302]