set expiration date for Tor Browser Developers signing key

I've noticed, that Tor Browser Developers signing key currently has no expiration date set.

gpg --fingerprint EF6E286DDA85EA2A4BA7DE684E2C6E8793298290

pub   4096R/93298290 2014-12-15
      Key fingerprint = EF6E 286D DA85 EA2A 4BA7  DE68 4E2C 6E87 9329 8290
uid       [ unknown] Tor Browser Developers (signing key) <torbrowser@torproject.org>
sub   4096R/F65C2036 2014-12-15
sub   4096R/D40814E0 2014-12-15
sub   4096R/589839A3 2014-12-15

I think it's recommended best practices to have one set.

Related to: #13407 (closed)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information