Optional 'parameters' in execute() method in database wrapper

The execute() method in the database wrapper now supports the
'parameters' argument as optional.
Added columns in 'suggestions' schema

Added 'title' and 'link' columns in the 'suggestions' schema.
Added 'View all suggestions' page

Added 'View all suggestions' page with jinja template support.
Changes that reflect new columns added to 'suggestions' schema

A little housekeeping, reorganizing a bit the flow of receiving a
suggestion.
'View all suggestions' page integration in mail file

Integrated the new page in the main Python file.