Fix UI bugs

• Fixed jumbotron text spilling into descriptions on the front page
• Fixed being able to propose projects outside of the proposal period on the front page
• Fixed misaligned button in the modal
• Fixed incorrect modal use throughout the application, leading to UI bugs
• Fixed the expansion animation on the front page by animating the text via CSS (perhaps it could be improved further by using only CSS for expansion rather than bootstrap collapsible)
• Added internal courses to the front page + notes for external users on the front page and on the offerer selection page
• Fixed an incorrect placeholder on the offerer selection page