Decode IMSIs found in Subscription-Id AVPs.

Move decode of the User-Name AVP out of the 3GPP-specific file.
Add a couple of macros for 3GPP AppIDs (rather than using the number directly)
in both files.

Change-Id: I496b4ae86b05264462167b6e41ce1451392de11b
Reviewed-on: https://code.wireshark.org/review/3753
Reviewed-by: Jeff Morriss <jeff.morriss.ws@gmail.com>