Problem: Multi-term search queries not tokenized into separate must clauses

### Current Behavior

Search terms entered in the search box are searching as a quoted string. For example, searching for `term1 term2` with `default_operator: "AND"` results differ from `term1 AND term2`.

Searching for `term1 term2` seems to be treated as a single string in one must query clause (`term1 term2`).

### Expected Behavior

According to the [AtoM Docs](https://accesstomemory.org/en/docs/2.9/user-manual/access-content/search-atom/#the-atom-search-box-searchbox) : "Search terms entered in the search box are, by default, searched with an “AND” operator - that is to say, searching for kitty cat would by default search kitty AND cat."

Searching for `term1 AND term2` should be parsed into 2 separate must queries (first for `term1` and second for `term2`) to the BoolQuery, to match the behavior of an explicit `term1 AND term2` search.

### Possible Solution

Parse the search query by tokenizing on whitespace and add each term as a separate must clause to the BoolQuery.

### Context and Notes

Bug in ES 6+

### Version used

AtoM 2.9+

Start by reproducing the difference between `term1 term2` and `term1 AND term2` on the Elasticsearch 6+ search path, then locate where the BoolQuery is assembled. Done means whitespace-separated terms become separate must clauses and behave like the equivalent explicit AND query.

Written by the indexing model from the issue text.