Patchable web vuln import

Pro requires the ability to patch the importing of '//web_vulns/web_vuln' elements from the exported XML because it outputs a format that uses Pro-only models and Pro-only modifications to Mdm::WebVuln.  report_web_vuln is already patched in Pro to handle the changes, but isn't being fed the correct arguments because [`Msf::DBManager#import_msf_xml`](https://github.com/rapid7/metasploit-framework/blob/a928e5f963199e394c8402a17674035bb616cd4c/lib/msf/core/db.rb#L3675) is monolithic and too long to replicate to patch just the [section that deals with Mdm::WebVulns](https://github.com/rapid7/metasploit-framework/blob/a928e5f963199e394c8402a17674035bb616cd4c/lib/msf/core/db.rb#L3951).  Therefore, I'll first break up `Msf::DBManager#import_msf_xml` so I can get a method that just deals with Mdm::WebVuln imports, [`#import_msf_web_vuln_element`](https://github.com/rapid7/metasploit-framework/blob/0bb79ba8904e0c0ecd17c1aba55ce2da118de6bf/lib/msf/core/db_manager/import_msf_xml.rb#L153), that I can then monkey patch in Pro to handle the Pro-only changes.