Automagically hide butter bars

[lesleyjanenorton]

This is a follow up to #146 because I overlooked the following acceptance criteria:
The warning disappears on its own when all proxy-setting extensions are inactive / removed.

Ergo, this PR:

• Automagically hides butter bar alerts when conflicting extensions are disabled and removed

  • Does not add these alerts to the list of dismissed alerts so that they are eligible to be shown again, should the same conflict or another conflict re-materialize

• Renames the fn exposed to the UI from removeAlert -> dismissAlert

  • Dismissed alerts are added to the dismissed alert list and are not shown again, regardless of the status of the conflict.

• Updates and adds tests