Migrate React layout components to Flow component syntax

[reosarevok]

Flow now supports a specific React Component Syntax which reduces the amount of types we need to define manually, and is supposed to be better for type checking as well. I'm converting our React components to this syntax bit by bit with this PR.

This changes all stuff in the layout subfolder.

Note: To be reviewed without whitespace changes (since this adds explicit return() calls that require spacing things one more tab in many cases).

On top of https://github.com/metabrainz/musicbrainz-server/pull/3228