Display explanation cards for assessmentCards / add explanation card global component

Feature Merge Request

Related Issue

resolves https://gitlab.com/fathom/app/issues/392

Description of Feature

• Add an 'explanationCard' global component that will be used both in the back of the assessmentCard and conceptCard

• Use it in the assessmentCard 'back', ie when you click on 'why' button

Motivation and Context

This came up when implementing clarifications on the back of assessment cards

What's Next?

• implement back of concept cards #391 (closed)
• implement correct text https://gitlab.com/fathom/app/issues/395

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