Implement NOT for search criteria

[simongray]

The current search functionality is quite powerful, but sometimes you probably want e.g. only letters of third parties mentioning someone without that person themselves being a correspondent in that exchange which is often the case. This requires being able to make search criteria like

[ mentioned: Daniel Jones ]
[ NOT correspondent: Daniel Jones ]

Ways to implement

1. Double the amount of options in the dropdown adding the negatives, perhaps with a heading. This is probably the easiest way to solve this, however, it might get unruly with that many options.

[simongray]

Things that are more difficult to solve:

• How should the NOT keys be represented in a way that allows for queries to be preserved in the URL?
• What about the _ (placeholder) symbol?

Perhaps the NOT part is better kept in the value part? e.g.

?document.recipient=!#np56&...

[simongray]

This is proving much more difficult to implement than I had first anticipated. Abandoning for now.

Work-in-progress changes can be found in https://github.com/kuhumcst/glossematics/tree/feature/65-not-criteria.