Add plate calculator

[michaelBelsanti]

I think a plate calculator similar to what Hevy has would be very useful. LiftLog already currently opens a popup when changing weights, so it'd be a fitting place to include it.

This is what Hevy's calculator looks like:

I think a more generic than specifying the bar weight could just be "starting weight" which could have preset options as well as allowing custom input for machines with nonstandard starting weights.