bsd.sites.mk: cleanup MOZILLA mirrors

• Switch to CDN by default as mirrors are no longer kept up to date
• Drop obsolete pointer to http://www.mozilla.org/mirrors.html
• Drop redundant BUGZILLA and MOZILLA_EXTEND
• Shorten MASTER_SITES in gecko@ ports
• Move MOZILLA_ADDONS to bsd.sites.mk
• Move one of MOZILLA mirrors with old addons under MOZILLA_ADDONS
• Addons CDN redirects to https://, so don't mislead with http://

https://blog.mozilla.org/it/2012/08/03/dear-mozilla-mirrors-thank-you/

Differential Revision: https://reviews.freebsd.org/D2550
Tested by: distilator
Reviewed by: mat (partial)
Approved by: bz-ports (ohauer), portmgr blanket (office@ et al.)
Approved by: portmgr (bapt, earlier version)
MFH: 2015Q2