Support LocationFallback on ILinkResources

[cafour]

Fixes #715.

This PR adds support for LinkResourceBase.LocationFallback during URL building and resolution of ILinkResources by adding a location query parameter to the dotvvmResource route.

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To illustrate the problem this PR solves, let's have the following resource registration:

resources.Register("FeatureSamples_Resources_LocationFallback", new ScriptResource {
    Location = new FileResourceLocation("~/Scripts/testResource_failsToLoad.js"),
    LocationFallback = new ResourceLocationFallback(
        "window.isTestResourceLoaded",
        new FileResourceLocation("~/Scripts/testResource_locationFallback.js"))
});

Without the query parameter, the generated URL of the fallback location is the same as of the primary location and thus it has no effect.

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I realize the main purpose of the LocationFallback property is having a backup plan for when a CDN is unavailable, and that a FileResourceLocation as fallback for another FileResourceLocation seems a little nonsensical but it is a valid configuration if only for testing the LocationFallback feature.

[exyi]

Is there any practical need for the location distinction? Surprisingly having multiple FileResourceLocations does not make much sense, since this fallback feature is designed to handle a failure to load a 3rd party resource. If we want to support failures to load local resource (due to missing file) I'd prefer to handle that server-side and not annoy the client-side about such issue.

[cafour]

The feature may have been designed to handle a failure in loading of a 3rd party resource but this intention is not enforced. The way it's designed (looking at the fallback condition) means it can be used to handle errors in a piece of javascript code. Having a fallback in that case could be useful.

Personally, I think we should support this scenario. But if we decide not to, I think, at the very least, we should notify the user that having multiple FileResourceLocations is meaningless.

(There's also my need to test LocationFallback in order to fix #702, but that can be solved another way.)

[cafour]

Merging after discussion with @exyi.