Support servers with subdirectory

[mikiher]

This adds support for connecting to servers with subdirectory, e.g. https://my.server.com/audiobookself.

This is the last step in preparing for subdirectory support, based on the proposed plan.

The required changes include:

• Allow paths when specifying the server address in the Connect form
• Fix this.player.src in AbsAudioPlayer.js (all other changes in this file are due to formatting)
• Connect to the correct WebSocket path
• Fix getLibraryItemCoverSrc and getLibraryItemCoverSrcById in globals.js

I tested this using the web client, as well as on my Android phone against a server running with RouterBasePath=/audiobookshelf (which should support both subdirectory and non-subdirectory access).

• I connected using the standard non-subdirectory address, and made sure everything was working
• I then connected using the subdirectory address, and made sure everything was working.

I did not test on iOS, as I don't have an IOS dev environement nor an iOS phone to test on.

[advplyr]

I tested this on Android/iOS/web including android auto. Works great, thanks!