Use btn-dark everywhere. Closes BiscuIT-ng#94.

biscuit/apps/exlibris/templates/exlibris/add_book.html

...uit/apps/exlibris/templates/exlibris/add_book_copies.html

biscuit/apps/exlibris/templates/exlibris/book_full.html

biscuit/apps/exlibris/templates/exlibris/copy_full.html

biscuit/apps/exlibris/templates/exlibris/edit_book.html

biscuit/apps/exlibris/templates/exlibris/edit_book_copy.html

biscuit/apps/exlibris/templates/exlibris/get_copy.html

biscuit/apps/exlibris/templates/exlibris/person_borrow.html