Unwrap testimonials in storybook and fix alphabetical sorting

[raphael-suter]

The story for testimonials should be top level like all other component stories.

Couldn't do it because of a strange storybook behaviour: when unwrapped, storybook moves the entire components section into a folder below 'OTHERS'

We should try again as soon as we have updated our storybook to 6.0.0 (#1774).

[raphael-suter]

Storybook doesn't want us to have top level stories. I assume it only works due to a bug in the storiesOf Api that we are using to create our stories and that's why we have this weird side effect.

I guess the update wont help.

Also have a look at #1900

[danyball]

new Story format CSF always group by title. But we want to have a flat list. (Is also related to the alphabetical sorting.) --> storybookjs/storybook#8255