#560 delete markdown, inline, contents worksheet items (no tables/bundles yet)
#560 Delete table rows and tables

- For Table bundles and Record bundles, (d) deletes the focused row, (ctrl+d) deletes the whole table
- Move shared table functionality into a mixin
- Clean up delete functions a bit
- get rid of isMac global var and just listen for ctrl or meta keys
Merge pull request #641 from codalab/feature/560-delete-worksheet-items

Feature/560 delete worksheet items