Allow project managers to change all meetings

Signed-off-by: Tobias Droste <tdroste@gmx.de>
Don't send notification mails on create/update

Signed-off-by: Tobias Droste <tdroste@gmx.de>
Add an option to select categories for meetings

There's a new option in the plugin settings which enables the
category selection list for meetings. This also enables different
css styles for entries with different categories.

To define which category should be displayed you have to edit
assets/stylesheets/meeting_calendar.css
and add a entry like
.fc-agenda .category_<cid> .fc-event-time, .category_<cid> .fc-event-inner, .category_<cid> a
{
  background-color: #<color>;
}
for every category.

<cid> is the category id which can be obtained by editing the
category. The URL contains the category id.
(e.g. http://[...]/issue_categories/1/edit means <cid>=1)

Signed-off-by: Tobias Droste <tdroste@gmx.de>