Warn user that changes will not be saved if s/he navigates away from JSON editor

ext.BookManagerv2.editWarning.js displays editwarning message on beforeunload if the input field values of the JSON editor form have been changed and user has enabled edit warning option in preferences.

Bug: 53449
Change-Id: I2e9137f98876d307c4ef62e74e7a3d579fedcdda
Merge "Warn user that changes will not be saved if s/he navigates away from JSON editor"