A "Clean up selection" button

[Ben]

Select the notes you want to quantize, then navigate to the browser console with f12 or ctrl+shift+i and use this function.

tweakNotes(n=>n.time = Math.round(n.time * grid) / grid)

You can learn more about console commands on the wiki at https://onlinesequencer.net/wiki/Console_Commands

ohh this is great, is there also something like this that works on the length of the notes? this only seems to alter their positions

No.  
However there may be a couple of tricks depending on your specific case.  

If a lot of your notes are meant to be mostly similar lengths, you can select them and drag the length all the way to the left, which will make them all the minimum length of the current grid.  Then you just have to do some minor fixing, which will probably be quicker than manually editing all of them.

You may also be able to make use of truncateNotesAt(time), which will chop the selected notes off at the designated time stamp (which is the number that appears in the url when you click somewhere on the measure bar).