Implement boolean queries in advanced search

Problem to solve

Currently, we don't do much in terms of relevance and ranking in GitLab's advanced search. There is some low-hanging fruit in implementing boolean queries to increase the likelihood that the most relevant search match is being displayed at the top of the results.

Proposal

Take the steps outlined in this Elasticsearch article to implement a strategy of utilizing multiple boolean queries, implementing a combination of:

• Default match query (OR operator)
• Match query with AND operator
• Match phrase query

Additionally, we should add a field boost (say, 2) to the match phrase clause.

Scope

We should do this for each of the searchable indexes in Elastic except code.