Show the Share button even when the code isn't running

首先跟踪现有的 Share 操作和 /compile 端点，以了解代码是如何发送和存储的。确定共享未运行或已修改代码时应如何工作，然后实现约定的客户端和服务器行为，使代码未运行时 Share 仍可用，并且无需不必要的编译即可共享预期的代码。

Suggested by @jbash in #1409. There should still be a Share button to share code even when the code isn't running.

In terms of implementation, this means that the code will have to be sent to the server when Share is clicked, since it hasn't been sent yet. Currently the only way to do that is to invoke /compile on the server. That is wasteful, though, so I should probably add a new server-side endpoint to store code without trying to compile it.

Open questions:

1. What if the current code has been modified since the program was run? Currently, we show a warning, and offer to re-run the program (which will then cause Share to share the latest code as well). Instead, if users shouldn't think of sharing as connected to running, maybe we should just ask them which they intended to share.
2. If this might cause more shares of incomplete code, should we stop users from accidentally sharing code that hasn't been run without source access? My sense is probably no... but it's worth considering if this would be confusing. The risk is that students will share something that they think works without testing, and then the person they are sharing with only sees a white box, and the student is frustrated because they never saw the error message.