Fix a typo and improve consistency in WP.org Profiles Activity Handler

[SergeyBiryukov]

Some inconsistencies in WPOrg_Profiles_Activity_Handler::handle_wordcamp_activity():

• The attendee_id branch​ uses string concatenation to append WordCamp date, whereas speaker_id​ and organizer_id​ branches above use separate strings for that.
• coming up %s seems to be typo, should be coming up on %s, as in the speaker_id branch.

[coffee2code]

In 3915:

Profiles Activity Handler: Improve WordCamp-related action strings.

• Add "on".
• For consistency, make attendee branch match string handling of the organizer branch.

Props SergeyBiryukov.
Fixes #1949.