Alllow changes to the SignalR hub location via Config

[KevinJump]

uSync leans on signalR quite a bit for UI and keeping the user informed of what is going on. it drives the nice icon progress bar on uSync. uSync will still work without signalR but it does lead to a less nice experience for users.

Recently while debugging an issue for a client we noted that the SignalR hub location had been changed by the UIntra package. and the package installed a modified uSyncHub.js file to change the location of the signalR hub loaded by uSync. While this works any subsequent updates to uSync break this link (by putting the orginal uSyncHub.js back).

to allow for people to change the signalR location but keep uSync working, we should add a config option that allows you to change the default location of SignalR for a site.