Fix the exception `AttributeError: 'Filter' object has no attribute 'block_url'`
introduced in this commit [1].
`self.block_title` and `self.block_url` were members of the Filter
object[2], but not anymore after commit [1].
This bug can be reproduced with setting WHOOGLE_CONFIG_BLOCK_URL to a
non-empty string.
[1] 10a15e06e1
[2] 284a8102c8