Refactor the styles used by the `HypothesisApp` component and other
top-level elements in the sidebar page to follow the conventions used by
other components. Also clean up some unused styles.

- Rename CSS classes for `HypothesisApp` component to `hypothesis-app*`
  to follow our normal naming conventions

- Consolidate styles for the `body` element in `sidebar.scss`

- Consolidate general element styles in `elements.scss`, eliminating
  `common.scss`

- Remove unnecessary styles for `html` and `body`

- Move `sheet` styles into `focused-mode-header` class, since these
  styles were only ever applied to that element

- Extract the styles from the `content` class used to center content in
  the top bar and content area into a mixin and then use that mixin in
  the component styles for the top bar and content area

- Remove unused mixin imports from `sidebar.scss`. These are no longer
  needed since the transition to the new SASS module system