Allow multiple assets in findings

[ramizpolic]

As as user, I would like to see only the unique findings grouped by assets when viewing the findings page.
When checking a specific finding, all assets linked to it will be displayed in a table instead of showing a single asset.

Tasks:

• docs: change asset-findings relationships rfc #1042
• Add AssetFindings models #1253
• Add AssetFindings controller #1257
• Use m2m relationship in Findings for Assets #1258
• Add aggregation logic to Findings #1259
• Use Findings with aggregation changes on UI #1260
• Support asset findings data on UI #1766