Make stackbuilder dependencies into a comma separated textfield

Make stackbuilder dependencies into a comma separated textfield

The "true dependencies" usign db relationships were too strict for
reality - multiple entries can have the same textid, and that's what
we need to depend on. Resolving it for platform is handled in the
StackBuilder client.

This update requires the following SQL to be run in the database:
DROP TABLE downloads_stackbuilderapp_dependencies;
ALTER TABLE downloads_stackbuilderapp ADD COLUMN txtdependencies varchar(1000) NOT NULL DEFAULT '';
ALTER TABLE downloads_stackbuilderapp ALTER COLUMN txtdependencies DROP DEFAULT;

Closes #167