Fix missing images everywhere when running locally

[YoshiRulz]

Before/after:

Only has effect in dev environments.

[YoshiRulz]

Build failure is here:

tasvideos/tests/TASVideos.Test/Pages/Wiki/RenderModelTests.cs

Line 18 in d72b31b

_model = new RenderModel(_mockWikiPages, _db, NullLogger<RenderModel>.Instance)

I'm not sure how to get an IHostEnvironment at that point. DI via a constructor parameter prevents the test from running.

[adelikat]

You have to mock it

[Masterjun3]

Is this the right place to insert a dev fallback? It relies on the broken links to be redirected to the wiki endpoint.
Instead, couldn't this be implemented more properly in the options when we add the UseStaticFiles. Maybe by adding a dev FileProvider that uses the regular file provider and doing the fallback logic there?