[HTML5] Add PWA support to the editor page.

This allows to install it as an app, and provide offline support (after
the first run).
Practically, this boils down to adding a JSON file as a manifest, an
offline page to be displayed when the cached files are not avaialble,
and a JS file to cache resources and return them.

The reason for the "first run requirements" is that some browsers, will
emit an "install" by just visiting the page (to see if the JS code is
compatibile), and we do not want to force casual visitors to just
download the 10 MiB+ compressed editor WebAssembly file without pressing
the start button.

Special thanks to Hugo Locurcio (Calinou) for the initial work.

platform/javascript/detect.py

@@ -7,7 +7,7 @@ from emscripten_helpers import (
     add_js_libraries,
     add_js_pre,
     add_js_externs,
-    get_build_version,
+    create_template_zip,
 )
 from methods import get_compiler_version
 from SCons.Util import WhereIs
@@ -147,12 +147,12 @@ def configure(env):
     env.AddMethod(add_js_pre, "AddJSPre")
     env.AddMethod(add_js_externs, "AddJSExterns")
 
-    # Add method for getting build version string.
-    env.AddMethod(get_build_version, "GetBuildVersion")
-
     # Add method that joins/compiles our Engine files.
     env.AddMethod(create_engine_file, "CreateEngineFile")
 
+    # Add method for creating the final zip file
+    env.AddMethod(create_template_zip, "CreateTemplateZip")
+
     # Closure compiler extern and support for ecmascript specs (const, let, etc).
     env["ENV"]["EMCC_CLOSURE_ARGS"] = "--language_in ECMASCRIPT6"