Bugfix: properly aggregate rule findings

[zimventures]

The current implementation of finding aggregation assumes that setting a member variable on a Django model reference will persist - it does not! This implementation update will ensure that the finding count and list of findings are aggregated for each rule, and passed back to the caller.

Verified by running a RegexRule that had multiple findings on a single asset.

[alex-nork]

Looks good 👍

There might be room for a small cleanup by making rule_dict a defaultdict, but definitely not necessary.

rule_dict = defaultdict(lambda: {'rule': None, 'findings_count': 0, 'findings': []})