Context: Toolbox can store key/value pairs in two places:- a settings store which is backed by a clear text json file per each plugin- native keystore for sensitive dataAt the same time some of Coder's clients (ex: Netflix) would like to deploy atscale preconfigured settings for Toolbox. Most of the needed settings are part ofjson backed store except the last used URL.This PR reworks the code around the last used URL/token and moves the URL in thejson backed store, making it easy to configure. At the same time we still supportthe pair stored in the native keystore for backward compatibility reasons.

After a successful connection to a deployment URL we usedto save the same token associated with two keys:- last-token- and associated with the last urlBut it does not make sense, once we have to URL we can easily resolve the tokenassociated with the URL. In other words this commit removes the ability to read/savea token associated with `last-token` key.

URL stored in the native OS secrets store is now superseded bythe URL stored in the JSON backed settings store.

With a takIf/let pattern. Theoretically you can have two threadsracing - one changing the value of lastDeploymentURL and another\one reading. In our code that should be pretty much impossible.But the compiler can't be sure of that so it throws and error.