mailer: support "on behalf of" registration (Fix #837)

(contains 6 new language strings)
Fix #839: Trigger mailer plugin when removing user from attendee list (back+front)
version set to 2.0.3, index.html added to xml files as needed
removed wrong update server url from xml and db tables (#834)
fix new mailer language strings (#837 related)
backend edit attendees send proper mails (relates #839)

In case admin changes an attendee unattend mails for old user and attend mails for new user will be sent.
(this requires sendemail field, so it's always shown)
backend: prevent doublicate attendee registration (relates #839)
Fix #845: Add Clear button and alter layout of backend edit attachments view (Event + Venue)
added 2.0.3.sql
Fix #850: always check access levels of logedin user
Fix #849: show attendances on open date events too
Fix #848: Remove CBE related things
Fix #838: adapt php extension check in installer, set release date to December
Fix #851: Check "Show attendee names to" setting on foreign CB profiles
Merge pull request #853 from jemproject/development

Fixes from development branch for JEM 2.0.3 - mailer, attendee, cb plugins