Add application setting to control shard number for indexing

Problem to solve

Currently the initial and incremental queues for Advanced Search indexing use 16 shards per queue (32 total). While this number works well for GitLab.com, it is likely too many for smaller self managed instances and can be reduced.

Proposal

Create a new application setting to control the shard size for indexing queues.

Default it to 2 or 4 for instances and 16 for GitLab.com