Add Searchable Property to Profile Fields

Currently, all fields in a profile are included in the profile search form. We want to give users the ability to control which fields are 'searchable'.

• Add is_searchable boolean column to uf_field table
• Add 'Searchable?' field to create/update profile field screen.
• When rendering search form for civicrm/profile screen - only uf_field's where is_searchable TRUE are included in search form.