Create table leadership type 42

[dmartin4820]

Fixes #42

What changes did you make?

• create leadership_type model
• register leadership_type in admin
• add leadership_type endpoints
• add leadership_type seed data
• add tests for leadership_type creation
• add test for leadership_type to project relationship

Why did you make the changes (we will use this info to test)?

• Here the seed data has an id column with blank id fields. This creates an empty column in the resulting .csv file used for creating seed data. To complete adding the seed data, I manually removed this column from the .csv file after dowloading from the Google sheet.
• Uncommented the leadership_type foreign key here. This is done so that the relationship is testable.