Mdm::WebVuln validations

Add validations to Mdm::WebVuln so that report_web_vuln patch in pro does not need to to check arguments, but can just rely on the validations like a conventional usage of ActiveRecord.  In order to test that the validations worked, I moved migrations from metasploit-framework/data/sql/migrate to metasploit_data_models/db/migrate.  This commit does include an additional migration that does not exist in metasploit-framework, so metasploit-framework also requires a pull request to use the new location of the migrations in `Msf::DbManager`.