Allow the user to start off with 3 projects [#13]

When the user visits a week page, they will be presented with at least 3
projects that they can enter or modify.

This ensures that the user starts off with 3 projects, which can be a
combination of new and existing projects that are attached to the
period.

Created the `Project` model and added the relationships between `Project` and `Period`.

This closes issue #13.