Fixed sidebar drag 'n' drop.

Was broken by refactoring in 18c0737d4e94e000d55d0a1b3fcdb21f00685da0.
Refactored editable table into HTML import.

The editable table element just extend's the table element's API, but
the rows to be used with editable tables have been moved into the import
too.
Moved editable table CSS into HTML import.
Refactored <plugin-menu>, <plugin-card> elements.

Part of #327.