Update jquery.tablesorter.js plugin (#235)

Update to the latest `v2.31.3` version of this `jquery` plugin, which
unblocks upgrading to `jquery` >= `1.9.x`.

Using a fork because:

1. The original repo hasn't been updated in seven years: https://github.com/christianbach/tablesorter
2. The most recent committer to the original repo ☝️  now maintains the fork.
3. The fork was updated just a few months ago.
4. The fork supports the latest versions of `jquery`:
     > Works with jQuery 1.9+ (`$.browser.msie` was removed; needed in the original version)

As far as I can tell, no other changes are needed.

This is a straight copy/paste of the non-minized code from: https://github.com/Mottie/tablesorter

I used non-minimized for simplicity, since this is a dev-only tool, the
perf impacts of minimization don't really matter.

I tested this manually by:

```shell
$ cd flask-debugtoolbar/test
$ flask --app basic_app run --debug
```

Then loading `http://127.0.0.1:5000/` in a recent version of Chrome and
finding a FDBT panel that uses the sorter. In this case, the SQLAlchemy
panel. I visually compared behavior pre/past upgrade and it appeared
identical.

Related:
* #166