Display current number of students upon welcome

[Carnagion]

When welcoming new students into the server, it would be nice to inform them how many students registered before them. A line such as You are the [insert number]th student to register! could be included as part of the welcome embed.

The displayed statistic may not be completely accurate, as users leaving the server (and therefore being de-registered) would skew the count. Nevertheless, the feature is not intended to serve an important purpose so this is OK.

This feature was present in the previous year's bot but not in the current implementation (neither in Rust nor Python), and would be a good way for new contributors to familiarise themselves with the project. It is thus left as an exercise to future maintainers :)