Add query parameters to search in transcription text and submission title

[TimJentzsch]

Relevant issue: Closes #212.

Description:

This adds the text__icontains query parameter to the transcription endpoint as well as the title__icontains query parameter to the submission endpoint. This enables clients (like Buttercup) to implement a search function.

Originally I intended to add the case-sensitive versions as well, but for some reason they were also case-insensitive.

Checklist:

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