update text welcome page
merge current, past activities in single category

Changing an activity post from category `current-activity` to `past-activity`
changes the url of the post, which makes permanent linking to an activity
impossible.  This commit moves all activity posts to the category `activities`
and adds `discontinued: true` to all discontinued posts.  `activities.html` is
updated accordingly.
add report of the kickoff meeting

closes #2