implement multi-select operations in collection management

[abartov]

In manage_collection view, allow toggling a multi-select mode that would allow, with either checkboxes or click-to-select-and-highlight, to select multiple items, including across sub-collections and including sub-collections themselves, and to then perform an operation on all selected/highlighted items.

The operations are:

• drag to a new location (using existing drag-and-drop visual implementation).
• delete (destroying the membership in the edited collection, i.e. the CollectionItems, not the items themselves from the catalog, of course)
• [in the future] feed the selected items into the batch-editing screen. (depends on UX for batch catalog editing #270 and on Allow batch-editing of meta-data of texts. #111.)

[damisul]

Hi @abartov
Here is a PR for a first part: added ability to select multiple items and do batch removal from collections/manage page: #454

As for batch dragging there are some issues...
As far as I see standard jquery-ui Sortable plugin does not supports batch dragging.

I've found some hacks in net to achieve this (e.g. https://stackoverflow.com/questions/5596748/sort-multiple-items-at-once-with-jquery-ui-sortable) and there is a third-party Multisortable plugin https://github.com/shvetsgroup/jquery.multisortable but I failed to quickly get them working. I see two possible areas for investigation:

• we use pretty complex html layout with a lot of scripting, so it can cause some issues (actually this is the reason why I've implemented multiselect on my own instead of using jQuery-UI selectable)
• those hacks and plugin were created several years ago, so there could be some compartibility issues related to browser/jQuery versions.

Not sure if we can resolve them quickly. Should I invest more time in multiitem-dragging support?