Add field for searchable data in the commit table

[osujipius]

This PR fixes mozilla/perfcompare#795

The commit table is modified to include a new field search_data that will be used to hold all searchable data related to relevant records

push = models.ForeignKey(Push, on_delete=models.CASCADE, related_name="commits")
revision = models.CharField(max_length=40, db_index=True)
author = models.CharField(max_length=150)
comments = models.TextField()
search_data = models.TextField(blank=True, null=True)