Remove "Extensions" page from Settings

[krschau]

Summary of the pull request

Remove the "Extensions" page from settings, in favor of navigation to extension settings through the Extensions page in the left nav.

References and relevant issues

#2212

Detailed description of the pull request / Additional comments

• Moved ExtensionSettings (page and viewmodel) to the Extensions project, since now that's the only place it is accessed by.
• Separate SettingViewModel into its own file separate from SettingsViewModel
• Remove logic to create a different breadcrumb bar depending on where we navigated from
• Remove special handling in page navigation
• Remove telemetry event sending which page was navigated from, since there's only one option now

Validation steps performed

Validated locally.

PR checklist

• Closes Remove "Extensions" from Settings Page  #2212
• Tests added/passed
• Documentation updated