Updated Member model and welcome template

Updated member model so that equity due is now
$25 by default. This update includes a hack to
the member.save() function so that now
equity_due is only cleared out when the
year of the date_departed is greater than
or equal to 1971, Mariposa's first year of business

Updated welcome template to include a message
informing members that they may not pay off
another account member's equity through PayPal, only their
own.
Merge branch 'master' of github.com:mariposacoop/mess