Add is_reachable field on the VulnerabilityAnalysis model #103

[tdruez]

Implementation for #103

Changes:

• Add new is_reachable field on the VulnerabilityAnalysis model as a tri-boolean field: yes, no, unknown
• Reachability column added in the "Vulnerability" tab.
• Add filter by "Reachability" from the column header.
• The is_reachable value can be set from the "Vulnerability analysis" modal form
• Add a VulnerabilityAnalysis REST API endpoint.

[DennisClark]

@tdruez the feature works very nicely in Staging Starship except for one problem I encountered with the filtering: Filters by All, Reachable, and Not reachable are working fine, but the Filter by Reachability not known returns everything (the same as All).

[tdruez]

@DennisClark Thanks for the review. The "Reachability not known" filter is now fixed.