From a very early time, we had a few things that Djblets monkey-patched

into the standard String class for JS code. For the most part these were

either unused or duplicated by library code:

String.trim() is a standard-library method that does what our

String.strip() method did.
_.escape() and _.unescape() do what our String.htmlEncode() and

String.htmlDecode() methods did (plus are slightly more complete).
String.stripTags() was only used in one place, which was already

  doing some regex transformations, so I've just copied the

  implementation of that into where it's used.
String.truncate() isn't used anywhere.

This change marks all of them as deprecated and replaces our

implementation with the third-party ones where we can.

Ran js-tests.