Enhancements and bugfixes

• Fixes falsy values displayed as NULL (#196).
• Sets correct content type headers when exporting schema to screen output
• Simplifies roles and privileges methods
• When inserting a new row, check the null input for empty fields that allow null values
• Avoid passing SQL in the query string when inspecting a column distinct values
• Export form has been extracted to a trait to reduce repetition (most of it is common to tables, views and matviews)
• Ensure server, database and table request params gets defined on hidden inputs when browsing a table