Add spinner + better attribution in REPL. Several changes to the REPL,
 - better attribution of Emscripten (we talked about it but didn't do the change): the gif below doesn't include this change.
 - Attribute for the Jquery Terminal: I added a clickable grey link on the right top of the REPL. On mobile, I think it should be OK, since we don't really scale text currently, so there is still enough space to display it. There was an issue that this was the only text that was displayed while loading.  
 - So I ended up adding a spinner for loading as well. Potentially we could re-use it for indicating that a package is being loaded as it's continuously running in the background, we just need to put it in the foreground with `z-index: 2` and hide it again after the package is loaded.
 
![Peek 2022-05-28 12-05](https://user-images.githubusercontent.com/630936/170820858-7f2ad6dc-0eb5-4f4e-9920-b59cfe8044e9.gif)


import pytest


def test_threading_import(selenium):
    # Importing threading works
    selenium.run(
        """
        from threading import Thread
        """
    )

    selenium.run(
        """
        from threading import RLock

        with RLock():
            pass
        """
    )

    selenium.run(
        """
        from threading import Lock

        with Lock():
            pass
        """
    )

    selenium.run(
        """
        import threading
        threading.local(); pass
        """
    )

    # Starting a thread doesn't work
    msg = "can't start new thread"
    with pytest.raises(selenium.JavascriptException, match=msg):
        selenium.run(
            """
            from threading import Thread

            def set_state():
                return
            th = Thread(target=set_state)
            th.start()
            """
        )


def test_multiprocessing(selenium):
    selenium.run("import multiprocessing")

    res = selenium.run(
        """
        from multiprocessing import cpu_count
        cpu_count()
        """
    )
    assert isinstance(res, int)
    assert res > 0

    msg = "Function not implemented"
    with pytest.raises(selenium.JavascriptException, match=msg):
        selenium.run(
            """
            from multiprocessing import Process

            def func():
                return
            process = Process(target=func)
            process.start()
            """
        )

import pytest


def test_threading_import(selenium):
    # Importing threading works
    selenium.run(
        """
        from threading import Thread
        """
    )

    selenium.run(
        """
        from threading import RLock

        with RLock():
            pass
        """
    )

    selenium.run(
        """
        from threading import Lock

        with Lock():
            pass
        """
    )

    selenium.run(
        """
        import threading
        threading.local(); pass
        """
    )

    # Starting a thread doesn't work
    msg = "can't start new thread"
    with pytest.raises(selenium.JavascriptException, match=msg):
        selenium.run(
            """
            from threading import Thread

            def set_state():
                return
            th = Thread(target=set_state)
            th.start()
            """
        )


def test_multiprocessing(selenium):
    selenium.run("import multiprocessing")

    res = selenium.run(
        """
        from multiprocessing import cpu_count
        cpu_count()
        """
    )
    assert isinstance(res, int)
    assert res > 0

    msg = "Function not implemented"
    with pytest.raises(selenium.JavascriptException, match=msg):
        selenium.run(
            """
            from multiprocessing import Process

            def func():
                return
            process = Process(target=func)
            process.start()
            """
        )
