Don't put <p>s in speeches which don't have a speaker.

And in fact, if there are <p>s replace them with <br />s.

Fixes issue #278.
Check that speech text is preserved on removing/adding a speaker.

A 'speech' with no speaker is represented in the Akoma Ntoso as
a <narrative> element, which can't contain <p>, whereas a speech
with a speaker is made up of paragraphs.

When someone changes a speech from having a speaker to not or vice
versa, we therefore have to add/remove <p> elements.

This test is to make sure that if we add a speech with a speaker,
remove the speaker, add them back again, and then remove them again,
that we always have the same two variants of the text.