Replace br tags with new line class for span elements

[ehanahamed]

<!-- ❌ 👎 -->
abc<br>
def

<!-- ✅ 👍 -->
abc
<span class="line">def</span>

br elements have some weird positioning problems that aren't consistent across different modern browsers, so we have <span class="line>...</span> instead. brs have already been replaced throughout Quizfreely's frontend code, but there's still a lot in EhUI.