Added a max_length setting to the originating share field

On the user register page and p2p register page, a PHP notice was complaining about an undefined `max_length` key. The originating share field from springboard social is an integer field, which uses the textfield widget to render. Textfield requires a max_length setting in its field config.

This pull request:
 - Adds the missing setting to the field definition in the code.
 - Adds an update function to add the setting to the field.

I set the max length to 10 as this field will hold a value from the share_id primary key, which is an unsinged integer of length 10.

This requires a DB update on existing sites.