Critical bug fixes and improvements to Gaps mode

- Gaps mode now asks for all words to be completed. Previously it would only remove nouns, verbs etc. From now, it asks you to complete the entire sentence. - Gaps mode now plays audio when clicking words - only if you have "Play on load" enabled in your auto play settings for the answer language - We now set the speaker voice to random for all new courses for all new users. As usual, you can specify specific speakers in audio settings - Smart mode now has a heigher chance of serving gaps questions in language courses for answers with more than 1 word - Improved reliability of auto playing audio based on settings - from time to time, especially with slow connections, it would still auto play audio for questions even if the settings was disabled - Critical bug fix: Difference in words was not showing up for new users or in certain scenarios - Critical bug fix: certain browsers would crash at the session complete stage