Add multiserver support for web client

[dkanada]

This enables multiserver support for the bundled web client for two reasons.

1. Why hide an option for no reason?
2. Some users may want this enabled.

This is exactly the same in Plex where you can select another server without leaving the current web frontend. I personally think this is useful, and it also works really well from the bundled frontend from my testing. I was able to switch between my dev and prod servers and the API calls were routed to the active server while the frontend calls were left on the current connection. I tested media playback, modifying user settings, and using the dashboard and everything worked fine for me.

[JustAMan]

LGTM.

Can you provide a screenshot what changes when we enable it on the web UI?

[hawken93]

I'd like this to be something that could be decided by configuration at a later point :) for now I understand it's far easier to just enable it. LGTM

[dkanada]

I want to make a few small changes before this gets merged to support running the web client on its own, should be done in a few days.

Dismissing my review as this is tagged as WIP now

[dkanada]

These changes will enable buttons to select a server in the user settings or navigation drawer for all web clients. All asset requests go to the hosted location regardless of the currently active server, and everything seems to be working fine. There shouldn't be any issues since this already works flawlessly on the Android client.

I have also implemented initial support for hosting the web client anywhere. I have been using nginx myself and it works great. The only change to get that working is adding the following two lines at the bottom of the index file. There is a single issue with a phantom server entry in the server selection page, but it has no effect on anything as far as I can tell.

<script src='scripts/apploader.js'></script>
<script>window.appMode='standalone';</script>

[JustAMan]

Mostly LGTM but one question.