Consider including version of CrowdAnki in config window

[aplaice]

It might be valuable for the current version of CrowdAnki to be available to the user, in an easily(-ish) displayable place, so that in case of issues they can easily say which version of CrowdAnki they're using.

Our current approach is to ask people to make sure that they're updated CrowdAnki, but occasionally AnkiWeb doesn't actual carry out updates (I remember at least one case where somebody was sure that they had updated, but AnkiWeb hadn't actually carried out the update).

If we want to include the version somewhere, then the config window is probably the most convenient and simultaneously least obtrusive place.

The main disadvantage is that making sure that the version number is included in builds might be fiddly. (Also, how do we handle beta versions/running from git etc.?)