Fix xpath and add stubbed mappings manager in specs

[aapomm]

Summary

The HTML importer is associating the issues in the first discovered node. This is happening because of the xpath we're using in the HTML importer:

html_evidence.xpath("//td[starts-with(.,'Host:')]").first

//td... will start a global search from the top level. We're calling #first afterwards so the first node in the document will always be selected.

Proposed solution
Use .//td... instead

Note that the PR also includes the following spec changes:

• Split the importer specs
• Stub the mappings manager service

Check List

• Added a CHANGELOG entry
• Added specs