Browser extension GSuite deployment fixes

Created by: lguychard

Rel #5604 (closed) Closes #3604 (closed)

Removes unused storage items, adds the ability to set the Sourcegraph URL through managed storage, and cleans up storage access.

• Remove unused storage items repoLocations (never read) and enterpriseUrls. The only consequence of having enterpriseURLs present in storage was to set the browser action to open the options page instead of the popup. In the options page, nothing specific is done when enterpriseURLs were present. They were otherwise unused in the browser extension.
• Handle reading the sourcegraphURL from managed storage.
• Split StorageItems by storage area: there are now different storage item types for local, sync and managed storage items. The typings of observeStorageKey and storage reflect this. Rather than copying all items from managed storage to sync storage upon installation, relevant storage items (for instance phabricatorMappings) are directly observed from managed storage.