Update field type in database when user requests to link/unlink selectbox

This change deals with the problem that arises when a user wants to change their drop down selectbox from a linked one to an unlinked one (where the admin supplies the options in the list).

When a drop down selectbox is initially linked, the data type for the particular field in the database is 'bigint'.  When the user wants to unlink, this data type needed to change to 'text', which previously was not happening. 

There were also some changes added for the opposite problem, where a user would have an unlinked selectbox that they wanted to link (and so the data type would need to change from 'text' to 'bigint').

The $linked flag was also removed, as it was no longer necessary.