Items dropped onto wrong zones in standard mode are immediately removed
from the board and sent back to the bank.

In assessment mode, items should stick in the dropped position until the
entire problem is explicitly submitted by clicking a button.

This commit only implements the "stick" part, but does not implement the
submit button and associated handler.

This commit also fixes an existing bug where the template function
would gett reused between blocks.

The template function (which is slightly different for each block,
depending on block's configuration) was getting assigned to the global
DragAndDropBlock function. Each block would overwrite the previous
value, which caused bugs.