Support multi-page sites

REPL.it does it, so why can’t we?

Popcode should allow coders to create sites with multiple HTML pages. Definitely a bunch of unanswered questions about how this should work, but a few parameters come to mind:

• Multi-page functionality should distract from the simple/common use case of just working on a single page. E.g. an always-on tab bar is probably not a good outcome.
• We only support multiple HTML pages. They all share the same CSS and JavaScript.
• By default, the preview shows the main HTML page. Coders can link to other pages (meaning the other pages will need “filenames”). We should probably have a validation to ensure that relative links point at valid “filenames”.

Unanswered questions:

• What happens when you’ve navigated to another page and then you start editing again? Does it keep you on the page you were on, or take you back to the index page? Repl.it takes you back, but it also doesn’t live-update.
• Do we need to build a back button? Does the browser back button work when navigating within the project preview? (Repl.it’s answers: no, yes)

Accommodating multi-page site workflows would be a major goal of the layout overhaul proposed in https://github.com/popcodeorg/popcode/issues/779

まず issue 779 で提案されているレイアウトの全面的な見直しと、既存のプレビューフローを確認します。CSS と JavaScript を共有する複数の HTML ページ、デフォルトのメインページプレビュー、および相対リンクが有効なファイル名を指していることの検証を完了の基準として定義し、実装前にナビゲーションと編集に関する問題を解決します。