Add filters for submission feed to transcription view

[TimJentzsch]

Relevant issue: Preparation for https://github.com/GrafeasGroup/buttercup/issues/202.

Description:

Adds additional submission__feed__* filters to the transcription view. This enables us to efficiently filter by subreddits for Buttercup commands and other functionality.

Checklist:

• [x] Code Quality
• [x] Tests (if applicable)
• [x] Inline Documentation