[fx53] Show compatibility warning if an add-on uses `-moz-calc`

[jvillalobos]

As part of bug 1331296, the CSS -moz-calc has been removed. All instances of this function should be flagged.

This is a compatibility warning, so the add-on should still be upgraded in this case. It should also appear as a warning in regular validations.

Message:

The -moz-calc function has been removed. You can use the equivalent calc instead. See this bug report for more information.

Affected add-on: https://addons.mozilla.org/addon/savefromnet-helper/