Fix TimeFilter backend for timezone aware times

[TimJentzsch]

Relevant issue: Closes #202.

Description:

There was a bug in the implementation of the TimeFilter backend that resulted in errors when providing timezone aware time strings as from or until parameters.

Specifically, it always used timezone.make_aware on the parsed time, which fails when the given time is already timezone aware.

Instead, the default timezone is now provided directly to the date parser.

Checklist:

• [X] Code Quality
• [X] Pep-8
• [X] Tests (if applicable)
• [X] Success Criteria Met
• [ ] Inline Documentation
• [ ] Wiki Documentation (if applicable)