Keep track of number of years of memebrship

[punchagan]

Currently, we only track if a player has membership for the current year or not. It would be nice to have first date of membership at least, if not the full history of membership.

This could be useful to automate things like -

• players who've been members for > 1 year need to have advanced certificates (current requirement)
• different membership fees for older vs newer members? (or some such other stuff)