Note re: Slickgrid version

While talking with @charles-cowart I realized that I wasn't actually sure which version of SlickGrid was in use (since the ["main" SlickGrid repo](https://github.com/mleibman/SlickGrid) stopped development almost 3.5 years ago).

After examining [our `slick.grid.js` file](https://github.com/jdereus/LabControl/blob/313d006ccad1d9111d218b7ce645eb6d10fe37a9/labcontrol/gui/static/vendor/js/slick.grid.js), as well as [this comment from Amanda](https://github.com/jdereus/LabControl/issues/354#issuecomment-426364528), I'm pretty sure we're using the [6pac community fork](https://github.com/6pac/SlickGrid) (which is still actively maintained).

@charles-cowart suggested I make this issue in order to:

1. just make note of this somewhere, because otherwise the next batch of LabControl developers will have to look this up
2. make a note for the future about updating SlickGrid to the newest available version (it looks like the 6pac fork is on v2.4 and we're on v2.3, so if desired we can update to that as a part of this sprint).