Add short version of membership description

- Added `membership_desc_short` to database
- House lists displays short version instead of long (helps with narrower screens)
- Use "Full", "Social", "Associate", "RA" instead of "Full Member", "Social Member", etc.
- Also updated how adding members parses the membership type to get the membership type value.
The exception wasn't being thrown here. It was thrown below when emailing imss.