refactor - Filters & Issues <3 rest API

Chunky MR.

I've refactored logic and exposed creating issues and filters via rest API. (All of the details can be found in commit descriptions)

The reasoning of all the changes was to:

• simplify current code
• split code by responsibility (aka make it more "Django"):
  • move validation/serializing code to serializes
  • move business logic to models.py (stats method from views to IssueFilter model)
• expose things via rest API

Short summary:

• new serializers for Bugs, IssueFilters, Issues, IssueFilterStats
• refactored endpoints:
  • convert to user query
  • issue filter stats
  • create issue filter
• new endpoint: create issue, hide issue, show issue
• remove some unused code
• fixed tool-tip style