Add ComponentAuthors table to database

[seesarahcode]

• Create ComponentAuthors table
• Add component_id column - integer
• Add author_id column - integer
• Components table should:

has_many :component_authors
has_many :authors, :through => :component_authors

• Authors table should:

has_many :component_authors
has_many :components, :through => :component_authors

• Add validations for component_id and author_id columns (presence)
• Create factories
• Write passing unit tests
• Create seed data