Create option for user defined report settings 

This creates an option to allow each individual user to configure receiving daily, weekly, or monthly reports instead of having a global only option. 

Details:
Added 3 new database columns in users table: daily_reports, weekly_reports, and monthly_reports.  Tested the standard command for upgrading ```rake snorby:update``` to make sure it adds these new columns automatically.  

The default makes no change and continues with the current behavior of sending daily/weekly/monthly reports on a global basis.  Setting ```user_reports_control: true``` in snorby_config.yml to true changes this and moves the checkboxes for "Daily/Weekly/Monthly reports" from the "General Settings" page to the "User Settings" page.