Add the ability to cancel a share scan

[jpdillingham]

I decided to tackle a bunch of stuff all at once:

• Adds an application-level cancellation token to help things shut down in a timely manner, especially when a scan is in progress
• Adds the ability to cancel share scans, along with an API endpoint and button on the UI
• Fixes a number of issues with share counts and state (reverses a bug that was added recently)
• Vastly improves the performance of the display of share configuration
• Forces a share scan if the application can't load the shared file cache on start (this also fixes the issue with share counts/status being unknown, as shares are either loaded, or scanned at startup)
• Updates the UI every 1% of scan progress, instead of 5% (this is all the monitoring I want to provide at the moment)

Closes #490
Closes #443
Closes #532