Series

- home page has link to create a new series if user is logged in
- after creating series, user is directed to the new series page
- series view has navigation buttons to help move to previous/next unit or first/last unit in the series
- if user is logged in, user can see in unit view and series view if they've completed units
- if user has completed all the units in a series, user can see a congrats line 
- added tables in database to contain table of completed units
- added table to contain list of series
- added table to contain what units are contained in which series and the order they were added