Saner alternative to tile redirect

Let's not do a 307 redirect for requests to `/tiles*`. It causes breakage with Ajax queries in Firefox and Safari. And it's just fairly messy.

Instead, let's send the tileserver base to the client with each survey. 

In the future we could send one big config package to the client (since the base is unlikely to change). However, that'd require a new endpoint, more tooling, tests, etc. Let's keep it simple for now.