Simplified configs, direct load, etc

* Did away with the concept of a static app_configs file and related
data. Unnecessary in practice
* Now leveraging localStorage to cache site_configs, which stores some
less time-sensitive values
* Introducing concept of data versioning for configs to know when to
refresh them from the source (based on listed plugin version, for now,
should be wp version if it ever makes it into core)
* Better JS defaults and syntax, avoiding potential crashes in some JS
engines.