Change event onboarding rainbow message to something more relevant. Default
message is considered as less user friendly than the new one.
Before this commit the category_id for a tag had no label displayed. We choose
to wrap the category_id field inside a group so that field labels shows up.
In settings, wrap the tag_ids inside a div.o_setting_right_pane so we can have
the left border displayed correctly, like most fields in settings.
Finally remove "Online" template from the event_data since it acts like a
dummy type without pre-defined data.
Task 2257884
closesodoo/odoo#52472
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>