Valid code microservice/endpoint with V8 Virtual Machine

Nobody has claimed this yet.

I've found some small bugs at the frontend when I try to run my own dweet, so I offer the following schema. We could create microservice or endpoint where will be created instance of V8's Virtual Machine with specified sandbox which is a canvas API stub.

So we could check valid our code or not, for example:

```
const isEqual = require('lodash.isequal')
const vm = require('vm');
const canvasStub = {
canvas: {
fillRect: function(){}
}
}

const sandbox = Object.assign({}, canvasStub)
vm.createContext(sandbox)

const code = "canvas.fillRect()"

try {
const result = vm.runInContext(code, sandbox, {
timeout: 1000
});
console.log('is code valid', isEqual(sandbox, canvasStub))
} catch (e) {
console.log('non valid code', e)
}
```

So if use will send `this.a = '1'` or `new XMLHttpRequest()` code will be not valid.
Also VM will have option for timeout that will protect against infinite loop in the code

In result if code is valid then we will create iframe with this user code

No repository files, tests, or entry points are named. Start by locating the existing frontend dweet execution flow and determine where a V8 VM-backed microservice or endpoint could fit. Done means the proposed sandbox validation handles the canvas stub, rejects the described mutations and XMLHttpRequest usage, enforces the timeout, and gates iframe creation on valid code.

Written by the indexing model from the issue text.