Use extend() from styled-components for internal use.

[igorschoester]

We currently use styled() from styled-components a lot. While they recommend using extend() for internal use. This only works if the Component itself is already styled. But should make it easier to overwrite styles later.

Files to change:

• [ ] composites/Plugin/Shared/components/Button

[igorschoester]

Extend does not seem to work like we thought... There are problems with specificity amongst others. Going to close this issue.