Make the "path" column a VARCHAR2(2048)

A path (the location of where a piece of anonymous feedback has been left)
is currently stored as an ActiveRecord `text` field, which maps to a MySQL `mediumtext`
field. In order to be able to put an index on this field, it needs to turn into a `varchar`.

The model validation on that field is a maximum length of 2048, and in the production instance,
the maximum length is 2030 chars. This makes it safe to change the column to be a `varchar2(2048)`.
Allow longer referrers

Before this change, `referrer` information for anonymous feedback
was being saved to a `varchar(255)` field. The rails 3.2 version of
the activerecord MySQL adapter truncated the referrer info, whereas
the rails 4.1 version raises an exception.

The `referrer` validation ensures that the field isn't longer than 2048 chars,
and the longest value in the production database is 2030 chars, so this limit should
be safe.
Merge pull request #21 from alphagov/adjust-database-field-limits

Adjust database field limits