Add project proposals into the projects widget

Resolves #346 (closed)

Adds the following:

• Include project proposals with the .featured-projects widget
• Ability to exclude proposals with the data-exclude-proposals data attribute replaced
• Ability to specify what project types you want to include with the data-types data attribute
• A util function getNextPage for traversing paginated api results with link headers, uses parse-link-header internally
• A helper function buildMockPaginationLinkHeader for building mocked pagination with link headers for tests