BUG: ignore exceptions in most_viewed template filter

most_viewed() template-filter search all `Submission` objects based on `pk` stored in `PageHit` objects. An exception is raised when the `Submission` object is not present in database but `PageHit` consists of its `pk` values.

PageHit model logs all data. So, the exceptions are ignored instead of deleting respective `PageHit` objects.
Merge pull request #155 from ksurya/pagehit-fix

BUG: ignore exceptions in most_viewed template filter