BRUSSELS — Britain and the European Union hailed a breakthrough Friday that allows them to begin talks on their post-Brexit relations without answering some key questions about how their divorce will play out. With pressure building on British Prime Minister Theresa May’s fragile government, the sides agreed on the last sticking point in Britain’s divorce terms: the border between EU member Ireland and the United Kingdom’s Northern Ireland. They both accepted that the border must remain open once Britain leaves the bloc in 2019, although they left it unclear how that would happen in practice.