Add relevant tag to domain document

[punisherVX]

As of 3.0, the tag is determined for each event as they come in. The tag should actually be associated with the domain beforehand as the lookup for the relevant tags are done for every event and this is not efficient. Will require rewrite of the domain doc class and logic to add the relevant tag at domain doc save time.

[punisherVX]

Each tag has now been written to the threat index for each event and is searchable and able to create visualizations based on the tag.