XDCR: Add support for inverting the filter

A couple of users have attempted to use negative lookahead in the filter regex to replicate all documents except those that match their criteria. This doesn't work, because Go's regex engine does not support negative lookahead.

We could support this use case by adding an "Invert match" checkbox below the filter. When this checkbox is enabled, XDCR would evaluate the filter, then negate the result (similar to how grep handles inverted matching).