Add an index on the logs_archived boolean on a RepositoryBuild

Addresses the slow query:

```
SELECT `candidates` . `id` FROM ( SELECT `t1` . `id` FROM `repositorybuild` AS `t1` WHERE ( ( ( `t1` . `phase` IN (...) ) OR ( `t1` . `started` < ? ) ) AND ( `t1` . `logs_archived` = ? ) ) LIMIT ? ) AS `candidates` ORDER BY `Rand` ( ) LIMIT ? OFFSET ?
```

While the cardinality on `logs_archived` will be low, it should also only be in the `false` state for a very small number of records, so it should make this query very, very fast.

data/database.py

@@ -997,7 +997,7 @@ class RepositoryBuild(BaseModel):
   trigger = ForeignKeyField(RepositoryBuildTrigger, null=True)
   pull_robot = QuayUserField(null=True, backref='buildpullrobot', allows_robots=True,
                              robot_null_delete=True)
-  logs_archived = BooleanField(default=False)
+  logs_archived = BooleanField(default=False, index=True)
   queue_id = CharField(null=True, index=True)
 
   class Meta:

data/migrations/versions/481623ba00ba_add_index_on_logs_archived_on_.py

@@ -0,0 +1,24 @@
+"""Add index on logs_archived on repositorybuild
+
+Revision ID: 481623ba00ba
+Revises: b9045731c4de
+Create Date: 2019-02-15 16:09:47.326805
+
+"""
+
+# revision identifiers, used by Alembic.
+revision = '481623ba00ba'
+down_revision = 'b9045731c4de'
+
+from alembic import op
+
+def upgrade(tables, tester):
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.create_index('repositorybuild_logs_archived', 'repositorybuild', ['logs_archived'], unique=False)
+    # ### end Alembic commands ###
+
+
+def downgrade(tables, tester):
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.drop_index('repositorybuild_logs_archived', table_name='repositorybuild')
+    # ### end Alembic commands ###