Refactor Global Quantity for Stock Entry

[jmcameron]

Refactor the various Stock Entry modes to handle Global Quantity more appropriately:

• If a stock entry is from a purchase order, requisition or shipment, then perhaps the global quantity should be locked and do not let the user change it.
• However, for entry from donations, integrations, etc (eg, there is no prior info on the available quantity), then we should probably hide and ignore the global quantity.

Rename 'Global Quantity' to 'Available Quantity'

See discussion about this in Issue 6638.