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. Testing Done: Ran js-tests.