Front-end Callback Improvements

[itsdawei]

Description

The front-end is a mess right now. This PR seeks to clean up the callback graph and data structures used on the front-end.

TODO

• [x] Clean-up the callback graph

Status

• [x] I have read the guidelines in CONTRIBUTING.md
• [x] I have formatted my code using yapf
• [x] I have linted my code with pylint
• [x] I have tested my code by running pytest
• [x] I have added a one-line description of my change to the changelog in HISTORY.md
• [x] This PR is ready to go