Show all button for activity collections > 3 items

This commit introduces a "Show all" button when an activity is a
collection of more than 3 activities. By default, only 3 tiles are
shown. When the user clicks the button, all remaining tiles are
displayed and the "Show all" button is removed.
Merge branch 'bootstrap3-large-activities' of https://github.com/simong/3akai-ux into bootstrap3-large-activities
Follow up on activity improvements
Small comment follow-up
Center user and group thumbnails
Self review on large activity items
Merge branch 'bootstrap3' into bootstrap3-large-activities
Followed up on code review
Merge pull request #3637 from nicolaasmatthijs/bootstrap3-large-activities

Activity widget improvements