Bug 1050416 - [Messages][Build] Add build script to ignore the desktop-only folder for production build. r=schung

* Added build script to delete desktop-only folder;
* Removed desktop-only "if"s from code;
* Changed way of loading desktop-only shims
I have mixed feelings. On one side, it's good enough for now. On another side, I fear we continue using ugly regexp for this if we need to add more stuff.

Here are some utils you might want:

Creates you a DOM in memory:
`var document = utils.getDocument(utils.getFileContent(filepath));`

and in `build/webapp-optimize` you can look at `serializeNewHTMLDocumentOutput` to write the document back.

Again, I'm not sure we need all this (especially the serialize part looks messy), I'd prefer if the build system would pass us a document and take care of the parse/serialize. We can maybe file a bug for this?