Backend for advanced search merge request source-branch facet

The source_branch field currently exists on the merge_request document

1. Pass down new param to SearchResults class
2. Add a new feature flag
3. Update MergeRequestQueryBuilder behind the FF to:

• if =: add filter for source_branch = source_branch
• if !=: add must_not for source_branch = source_branch

Params will be expected from the frontend as:

• source_branch and not[source_branch] (translated to not_source_branch to be sent to Elasticsearch)