feat(web-server): Optionally serve JS concatenated

With the option 'concatenate: true', all JavaScript is concatenated into
one JavaScript file that uses eval and the //# sourceURL=... trick to
load JavaScript in one request while maintaining debuggability.

This has substantial performance benefits with many small JavaScript
files (hundreds) and allows transforming JavaScript sources to handle
specific module loaders (e.g. goog.loadModule).
feat(web-server): Support goog.loadModule.

goog.loadModule works exactly like eval() based concatenation, except
that it requires a call into the special function goog.loadModule.