Replace user_blocks fixture with a factory.

Traits are used here to make the tests easier to read, and the
different traits can be combined where necessary.

test/fixtures/user_blocks.yml

@@ -1,29 +0,0 @@
-active_block:
-  id: 1
-  user_id: 13
-  creator_id: 5
-  reason: "Active block"
-  reason_format: "markdown"
-  ends_at: "2032-05-01 11:22:33"
-  needs_view: true
-  revoker_id:
-
-expired_block:
-  id: 2
-  user_id: 14
-  creator_id: 15
-  reason: "Expired block"
-  reason_format: "markdown"
-  ends_at: "2012-05-01 11:22:33"
-  needs_view: false
-  revoker_id:
-
-revoked_block:
-  id: 3
-  user_id: 13
-  creator_id: 15
-  reason: "Revoked block"
-  reason_format: "markdown"
-  ends_at: "2012-05-01 11:22:33"
-  needs_view: false
-  revoker_id: 1