galaxy-reload-safetybelt -> reload-safetybelt
Make more generic `additionalStaticJs` interface in webapp.

Now webapp doesn't need to know anything about the reload safetybelt; it
just exposes a generic interface that any package can use to include
additional static JS (inlined when possible) in the boilerplate
HTML. reload-safetybelt uses this interface, instead of webapp
weak-depending on reload-safetybelt.