Reorganize files and extract common components

[MrOrz]

This is about cleaning up and pave way to the ArticlePageLayout refactor PR (#288 ).

Although there are 30+ files changed, a majority of them are caused by moving files around.

Features

• Add storysource to Storybook
• Mock date on snapshot -- not used in this PR, but is essential in Replace <ArticleItem> with more reusable card components #288.

Refactor

• Rename components/ListPage to components/ListPageControls
  • This is to differentiate from the controls (inputs) from display (output) components.
• Move display component for list pages to components/ListPageDisplays
  • components moved: ArticleItem, ReplyItem, ReplySearchItem
  • actually all of the above items will be replaced by re-encapsulated, reusable components in Replace <ArticleItem> with more reusable card components #288 . But the scope of "ListPageDisplays" remains the same.
• Remove ArticleItem dependency in article detail page (page/articles)
  • Article detail page does not directly use <ArticleItem>, thus we should not add its fragments
  • In article page loading fragment, we replaced ArticleItem's fragment with fields that is actually used.
• Removes obsolete files
  • components/Modal: not used anymore, replaced by Material-UI dialogs
  • componnets/__snapshots__/TimeRange.stories.storyshot: obsolete snapshot that we forgot to delete previously

Screenshots

No visual change at all.