Allow linking direct to signup for specific membership types

[proffalken]

Is your feature request related to a problem? Please describe.

At the moment, all links from our website point to /register, and then the new member has to select the level of membership they require when they get to the appropriate screen

Describe the solution you'd like
A way to "autoselect" the membership level as part of the signup link such as /register?membership_type=full_member or /register?member_type_id=3`

Describe alternatives you've considered
Continuing as current with no customiseable links