TestArtifacts support to backend

Summary:
	-create testartifact table
	-parse test artifacts xml data for each test output (in XunitHandler) and store it in the testartifact table

Test Plan: unit test for parsing the xml test artifacts data

Reviewers: vishal

Reviewed By: vishal

Subscribers: cramer, vishal, changesbot

Projects: #changes

Differential Revision: https://tails.corp.dropbox.com/D75785