feat(nimbus): Update nimbus experimentation code

[vbudhram]

Because

• We want to reenable Nimbus experimentation in accounts

This pull request

• Moves Nimbus initalization from AppContext into NimbusContext and NimbusProvider
• Updates Nimbus configs, adds enable flag
• Removes unused code

Issue that this pull request solves

Closes: https://mozilla-hub.atlassian.net/browse/FXA-12558

Checklist

• My commit is GPG signed.
• If applicable, I have modified or added tests which pass locally.
• I have added necessary documentation (if appropriate).
• I have verified that my changes render correctly in RTL (if appropriate).

Other information (Optional)

We will need webinfra PRs to add the updated Nimbus Cirrus servers