terms: make filters togglable

If a filter has already been created for a particular term label, remove
it instead of adding another one.  This makes it convenient to
add/remove term filters quickly without needing to use the filtering
panel.
Terms option to ignore it's own filters
Allow Terms panel filtering mode to be configured to 'must' or 'either'
Merge branch 'terms-ignore-filters' of https://github.com/skillflip/kibana

Conflicts:
	src/app/panels/terms/module.html
	src/app/panels/terms/module.js