Refactorings, cleanup, etc.

[PzYon]

• ViewItemPage and EditItemPage could both be wrapped by a DoWithItemPage or something
• Rename common-folder to reusable, e.g. move Markdown.tsx and CodeEditor.tsx there
• Replace constructors with public readonly state: I..State
• Create ActionButton or something for anchors (reason: to have a unified "action" component, a11y)
• Analyze usages of inline function, especially in for/map (reason: performance)
• Run Audit in Chrome - see if something interesting is printed
• Consider adding react-hooks eslint plugin