Create a submodule to only index issue nodes and their comments to the core search index

Based on #377270: Core search index not complete and a general desire among the infra team not to maintain the core search index for all nodes, we discussed just indexing project_issue nodes and their comments for use on the issue advanced search views. Most sites could just rely on the core search index (assuming it's complete), but for sites that are using something else for general search but still want issue views, we can provide this sub module for their needs. Damien is apparently finishing a first draft now, just starting an issue about it to track the code and progress.