onStartMoving() fix to minHeight (#999)

[adumesny]

Description

*method was not correctly counting margin when figuring out minHeight during a drag
(see #999)

• fixed cellHeight() getter to correctly returns the value and checked all places that use to adjust for margins

Checklist

• All tests passing (yarn test)
• Extended the README / documentation, if necessary

I tested two.html as well as my app that doesn't a lot of drag&drop and made sure things resized correctly and no new issue, but min size is now correct (see #999 screen shots)

[coveralls]

Coverage decreased (-0.2%) to 65.583% when pulling 6b07c28 on adumesny:bugfix/999 into 66339a5 on gridstack:develop.

[radiolips]

opts.verticalMargin can be a string, so those references will need to be changed to something that returns a number.

[adumesny]

@radiolips I changed one of the demo (two.html) to have verticalMargin: "2rem" in the opts and sure enough gridstack constructor calls this.verticalMargin(this.opts.verticalMargin, true); and the only other way to set is to call verticalMargin(val) directly - in there we do extract the number vs the unit so opts.verticalMargin will NEVER store a string.

Now mind you we fail to convert the units to actual pixels so '2rem' = '2px' = '2em' = 2 which is clearly incorrect, but that's a separate issue (maybe the CSS is update for other values, but verticalMargin is pixel code and therefore wrong.