Adding, modifiying, and/or deleting questions for an already started
poll is far away from being democratic and can lead to unwanted side
effects like missing votes in the results or stats.
So, from now on, only modifiying questions will be possible only if
the poll has not started yet.
As mentioned in commit 5214d89c8, using a `<select>` tag which
automatically submits a form on change has a few accessibility issues,
particularly for keyboard users who might accidentally submit the form
while browsing the options.
So we're adding a submit button and removing the "submit on change"
behavior.
Note that, while `<select>` tags have their own usability issues,
alternatives in this case are not obvious because the number of existing
polls could be very low (zero, for instance) or very high (dozens, if
the application has been used for years).
I thought of using a `<datalist>` tag with a regular text input. The
problem here is we don't want to send the name of the poll to the server
(as we would with a `<datalist>` tag); we want to send the ID of the
poll.
Maybe we could add an automplete field instead, providing a similar
funcionality. However, for now we're keeping it simple. This poll
questions page isn't even accessible through the admin menu since commit
83e8d603, so right now anything we change here will be pretty much
useless.
