make some URLs at dashboard gettexted

[tai]

Please make some URLs below at dashboard gettexted:

'dashboard_primary_link', '​http://wordpress.org/development/'
'dashboard_primary_feed', '​http://wordpress.org/development/feed/'

'dashboard_secondary_link', '​http://planet.wordpress.org/'
'dashboard_secondary_feed', '​http://planet.wordpress.org/feed/'

I know I can change the URLs at dashboard since 2.5, but now some local teams have own sites on wordpress.org, so it would be nice if we can change those default URLs with .po files.

[hansengel]

+1. Language files translate these URLs to wherever the support sites are in their language. These should be gettexted.

[ryan]

(In [7292]) Mark dashboard feed links for translation so they can be localized. Props tai. fixes #6222