Support adding arbitrary links to token images

[samcf]

At the moment, there is currently no way to associate a token image to an external resource with details about the character or creature it represents. This feature would extend token images to support a URL that users would enter themselves, such as a link to a DNDBeyond page or Google sheet.

• New controls to edit the URL for token images in the tokens panel.
• New controls to edit the URL for individual tokens.
• New controls to open the token URL (if it exists) in a new browser tab where it makes sense.
• New shortcut (⌘ + K / Ctrl + K) to access those controls for tokens.