Issue 0004843: Implemented improved text filtering

The free text filter box now searches for individual terms in the field
separately, obeys quoting patterns, and allows for negation. All terms
found are required to be found somewhere in the issue (or not found
anywhere for negated terms) in order for the issue to match the filter.

Example searches and their expected results:

bus car
Should find any issue that mentions both "bus" and "car"

"micro computer"
Should find any issue that mentions "micro computer" exactly; this
matches the existing search behaviors

apple -banana
Should find any issue that mentions "apple" but has no mention of
"banana"

circuit -"power line"
Should find any issue that mentions "circuit" but has no mention of
"power line"