Implement theme-clean variants of card pattern

[lyzadanger]

There are some "clean-theme" affordances in the client version of the "card" pattern styling, which is used by several components, including panels and dialogs. Those affordances need to be understood, reproduced here and represented in pattern library examples of those patterns and components.

This needs to be done before wholesale replacement of Panel and Dialog components across various applications with shared versions.

Part of #83

[lyzadanger]

In the client, the current card pattern conflates two things:

1. A generic card-like pattern that is re-used by panels, etc.
2. A specific pattern for annotation cards — a set of vertically-listed cards in the sidebar

We need to address clean-theme affordances for the first item here now. Specifically, that is:

• removal of any box-shadow and hover effects and
• removal of borders

When we pattern-ize annotation cards, we'll need to address the second item: putting extra space between annotation cards and a "separating" border between the cards.

The card pattern in the client is defined here: https://github.com/hypothesis/client/blob/7563a60c553b271da88c081dc93974071a07dda7/src/styles/mixins/molecules.scss#L18-L47