Add ability to add/edit custom blocks from the sidebar management grid

[ajnyga]

At the moment new blocks are added from website=>plugins=>customBlockManager settings.

It would make more sense if adding/editing blocks would be possible from the sidebar management grid. Basically an "edit block" link in the upper right corner of the grid panel would be enough.

https://forum.pkp.sfu.ca/t/ability-to-add-edit-custom-blocks-from-the-sidebar-management-grid/33633