Table improvements

Part 3 in our recurring series of improvements to tables in Outline. This release includes the following updates:
- Table columns and rows can now be reordered by dragging the column or row header to a new position
- Table cells can now have a custom colored background, use the new "Cell background" option in the toolbar to choose a color
- Sorting of table data is more natural for currencies, dates, and numbers
- Lists inside tables no longer break Markdown output
- Tables with merged cells now behave correctly when sorting
Other improvements
- Added the ability to choose custom color highlights for text
- Clicking on a group in the share dialog now shows it's members
- Added a hide/show completed items control for checkbox lists
- Added Alt-click to recursively expand or collapse sidebar documents and collections
- Added a preference for desktop notification badge with off, count, and indicator options