ENH: order search results based on wilson score

1. New `score` field added to Revision model, which stores wilson, score
confidence
2. allvotes_count, upvotes_count model methods - only count when
is_valid=True
3. add new field to haystack search_indexes
4. Filter "tag" search based on score
ENH: show top-rated in front-page

1. create another tab 'top rated' on front page to show submissions based on score! Filter is created for it
Note: only revision having top Wilson score is showed

2. show all submissions based on score
The above "note" is applicable to [2]

3. create model property in Submission model to get top_revision of the respective submission
ENH: enable reputation for user profile

1. `reputation` field in user profile is present before but never used!
2. reputation for user profile is done as soon as successful voting is
done