CC2 font change and presentation gaps

Today we made two minor tweaks to the online test presentations.

The CC2 words now use "Arial" font face to more closely match the printed tests. Previously it was using a default "sans" font which could be different in different browsers or operating systems, (e.g. Helvetica is preferred by Mac as the default "sans serif").

The CC2 and all other online test stimuli now disappear briefly between presentations to help separate them. It was noticed that when some of the stimuli were swapped to the next one, the presentation moment was difficult to notice especially in cases where the words are similar.