Provide an AND / OR option to advanced search

Currently with Advanced search all the clauses are AND'ed together. We should give folks the options to "OR" the clauses. Most of the times each clause is a form element.

Also allow this "Search Operator" to be sent in via the url, so admins can create navigation menu items. Default operator is AND. Add "&operator=OR" to change the default operator to OR via URL.

This will apply across all accordion tabs, including custom fields. Thus if a user has selected:

Groups G1, G2
State: CA
Country: US
Custom Field 1: '123'

we will return all contacts that satisfy the clause:

AND operator: ( In Groups G1 OR G2 ) AND State = 'CA' AND Country = 'US' AND Custom Field 1 = '123'

OR operator: ( In Groups G1 OR G2 ) OR State = 'CA' OR Country = 'US' OR Custom Field 1 = '123'

Note that the groups, tags and other multi-select and most checkbox elements like State, Activity Type, Activity Status etc. are single elements, and their values are always OR'ed

A few other changes:

1. Number ranges (e.g. contribution amounts) and Date ranges (e.g. contribution dates) are always AND'ed irrespective of search operator

2. we suppress the is_test clause for Seach operator for test contributions / participants OR (ideally we'd like to set it so that it is included as an AND)