Minor fixes to sponsorship packet stuff

[Tookmund]

Looking to redeploy this as we begin to ramp up for our next hackathon, I realize I never submitted these fixes upstream, so I'm doing that now.

Firstly, it seems there was code added to shared/packet.py to handle getting the packet file path, but it wasn't used when attaching the packet to the email.

Second, I set a default value for the location of the sponsorship packet in website/static so that I didn't have to set this when I was just getting the packet from a URL. This allows for easier first-time configuration, but also leaves the current level of configurability if desired.

[jwoglom]

Thanks for the fixes! Please feel free to send more improvements upstream :)

[Tookmund]

If/when this is merged, there's a minor documentation update that should come with it
https://github.com/ACMWM/hackerforce/wiki/Environment-Variables

diff --git a/Environment Variables.md b/Environment Variables.md
index 8bcd71e..07c50ce 100644
--- a/Environment Variables.md  
+++ b/Environment Variables.md  
@@ -45,4 +45,4 @@ does not exist in the filesystem.
 The local name of the sponsorship packet, to be stored in the website/static
 folder. If this file exists, it will be used. Otherwise, it will be
 re-downloaded from `SPONSORSHIP_PACKET_URL`.
-Must be set regardless of whether `SPONSORSHIP_PACKET_URL` is set.
+It defaults to `sponsorship.pdf`.