Unify loading animations across the system, fixes #930

app/views/messages/overview.php

public/assets/images/loading-indicator.svg

resources/assets/javascripts/bootstrap/application.js

resources/assets/javascripts/bootstrap/files.js

resources/assets/javascripts/bootstrap/members.js

resources/assets/javascripts/bootstrap/statusgroups.js

resources/assets/javascripts/bootstrap/subcourses.js

resources/assets/javascripts/lib/admission.js

resources/assets/javascripts/lib/course_wizard.js

resources/assets/javascripts/lib/fullcalendar.js

resources/assets/javascripts/lib/resources.js

resources/assets/javascripts/lib/smiley_picker.js

resources/assets/javascripts/mvv_course_wizard.js

resources/assets/stylesheets/scss/admin.scss

resources/assets/stylesheets/scss/ajax.scss

resources/assets/stylesheets/scss/globalsearch.scss

resources/assets/stylesheets/scss/installer.scss

resources/assets/stylesheets/scss/progress_indicator.scss

resources/assets/stylesheets/scss/search.scss

resources/assets/stylesheets/scss/smileys.scss