Add Internal Models For Notification Contents and Video Channels

[thealphadollar]

Fixes #

Short description of what this resolves:

• Adds two internal models (not exposed via API).

Changes proposed in this pull request:

• add: notification_contents internal model
• add: video_channels internal model

Checklist

• I have read the Contribution & Best practices Guide and my PR follows them.
• My branch is up-to-date with the Upstream development branch.
• The unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

• All the functions created/modified in this PR contain relevant docstrings.

[norbusan]

THanks, that looks nice, but I think it would be better if the introduction of the basic model is paired with the serializer, the views, and the URL definition, not to split all of it apart.

WDYT?