Add rights grant agreement to registration form (fixes #378)

[Keltena]

This pull request:

• Adds a checkbox to the entry registration form as specified in Add a checkbox to the comp submission form reaffirming Rule 2 for Authors #378:

• I hereby grant the Interactive Fiction Technology Foundation (IFComp's organizing body) the non-exclusive right to distribute, without limit, all material I submit to the competition.

• Disables the submit button unless the box is checked.

(It does not currently do anything else, such as display a warning that the box isn't checked or record the fact that the box was checked.)

[coveralls]

coverage: 65.299%. remained the same
when pulling a86f767 on Keltena:rule2-confirmation
into 509996f on iftechfoundation:main.