Use security.ssl.disable_session_identifiers pref in meek-http-helper to restore TLS session tickets

comment:1:ticket:13442 shows that meek-http-helper in Tor Browser 4.0 doesn't match stock Firefox because it is missing the TLS session ticket extension. It's because of #10822 (moved) (and upstream Mozilla #917049 and #967977) which replaced one pref with another.

I've tested manually that setting security.ssl.disable_session_identifiers=false restores the missing extension. A test build with this patch has just started, but it's straightforward enough that I'll attach it now.