Split up serializer into multiple files.

* Split into a few big chunks:
  * serializer state in one file
  * all tag handlers in one file
  * link handler (since this one is particularly hairy) and
    image handler in one file
  * separator functionality in one file
  * wikitext escaping functionality in one file.
  * utils for functions used across files.

* Renamed dom.minimizeWTQuoteTags.js to wts.* to use the same
  naming convention as the rest of WTS related files.

Change-Id: Ifbfa042ceaa0228c481a52b855a2a2aa1f615344
Merge "Split up serializer into multiple files."