Navigator should be able to load content files that have dangling references

[ElJocko]

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A content file may have dangling references, that is, references to objects that are not in the file. This is particularly the case when a matrix references a tactic (stix.tactic_refs[]), a technique references a tactic (stix.killchain-phases[].phase-name) and a data component references a data source (stix.x_mitre_data_source_ref). These references are included in the source object directly, whereas all other references are set by using a separate relationship object.

The current version of Navigator fails when a content file does not contain all the tactics referenced by the included matrix.

The Navigator should successfully load a content file that contains dangling references. When loading and displaying data, the Navigator should ignore any reference where the referenced object is not included in the content.