Change the outcome per threshold bar chart to area chart

[JosephMarinier]

Resolve #430

Description:

• Clean up front-end imports

• Clean up get_dataset_split() by leveraging super()

• Compute outcome count at 100% threshold

• Return error if post-processing is not available

• Draw area chart

  • Add confidenceThreshold and utteranceCount to API response
  • Change the outcome per threshold bar chart to area chart, making the x axis continuous

• Add a dashed line for current threshold

• Log changes

• Adapt test to error if post-processing is not available

Checklist:

You should check all boxes before the PR is ready. If a box does not apply, check it to acknowledge it.

• ISSUE NUMBER. You linked the issue number (Ex: Resolve #XXX).
• PRE-COMMIT. You ran pre-commit on all commits, or else, you
  ran pre-commit run --all-files at the end.
• USER CHANGES. The changes are added to CHANGELOG.md and the documentation, if they impact
  our users.
• DEV CHANGES.
  • Update the documentation if this PR changes how to develop/launch on the app.
  • Update the README files and our wiki for any big design decisions, if relevant.
  • Add unit tests, docstrings, typing and comments for complex sections.

[gabegma]

LGTM for the back end!! Good job! Some minor comments for the raise, but I'm fine with the current implementation too if you think that it's preferred.