[FEATURE] Allow mass removing definitions added by Migaku

[KieranBrannigan]

If some kind of wrapper tag is added to the definitions then this would allow mass removal of definitions generated by the extension.

For example, if I am mass generating definitions for 500 cards which already contain some information in the definitions field, but it fails halfway through, I have no way of undoing the half-done job and retrying.

Another example might be mass removing all definitions and replacing them with monolingual definitions.