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

首先审查 issue 779 中提出的布局全面改造方案以及现有的预览流程。围绕多个共享 CSS 和 JavaScript 的 HTML 页面、默认的主页面预览，以及验证相对链接是否指向有效文件名来定义完成标准；在实现之前解决导航和编辑相关的问题。