Also, fix for notifications throttle delay value

This paves the way to a more effective search in corpus by modifying the
internal "leaves" type of the `Query` type from a `Term` to a new type
called `QueryTerm`, which can either be an extact match or a partial
match, which can be rendered into a proper Postgres TS query via the
":*" syntax.

Tests were written with a very flat module hierarchy which made them
a bit confusing to navigate. This commit moves everything under a common
`Test.*` hierarchy.

This commit also uses a temporary fork of boolexpr.
It allows to parse certain terms like "Niki" without choking.

This commit adds a bunch of Arxiv tests to demonstrate we can
successfully transform our `Query` type into the Arxiv AST.