Support for PUT and DELETE API requests

* For PUT and DELETE requests, the request body is
encoded to JSON by default and the content type for the
request header is set to "application/json;charset=UTF-8"
Drop comments and links

* Implemented creation of new links via the "Edit" modal
dialog when the drop detail dialog is open

* Commenting on drops from the drop detail view
Simultaneous update of metadata views

* When a metadata item is added/deleted, the side listing
on the drop's full view is updated and so is the counter next
to the "gear" icon
Add drop tags

* Implemented adding of tags - with a drop down to select the tag
type - via the dialog used for editing a drop's metadata