Bug 35062: Add class plugin-blocks-submit in addbiblio

Bug 35062: Add class plugin-blocks-submit in addbiblio

Adding this condition in Check() allows you to prevent form submittal
during e.g. an ajax call in a framework plugin.
The plugin adds and removes the class on the corresponding html
element during processing.

Test plan:
Since we do not have a plugin using this class yet, nothing changes now.
Verify that you can still click Save in addbiblio while using standard
plugins. In other words: no change in behavior.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>