1-M: Asset Group >- (Primary) Tool

## Overview
As a user I want to know what tool is the primary tool for an asset group

### Action Items
- [ ] Update asset group models.py to add field primary tool which should be a foreign key. Look for other foreign keys.
- [ ] Run source ./start-dev.sh. This should create a migration script to add the foreign key and then run that script.

### How to Manually Verify
- From the asset group screen verify you can associate with a (primary) tool with the assset group.

### Automated Tests

### Resources/Instructions

Start in asset group models.py and inspect existing foreign keys to determine how the primary tool field should be represented. Run source ./start-dev.sh to generate and apply the migration, then use the asset group screen to verify that a primary tool can be associated with an asset group.

索引モデルが issue の本文から書いたものです。