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)


package:
  name: demes
  version: 0.2.2
source:
  url: https://files.pythonhosted.org/packages/2f/e4/86796d77ea453aa96f1ce758b461cdf738177f441a18c832cc9f29848903/demes-0.2.2-py3-none-any.whl
  sha256: 54f0110e9f1b5167347fb4793f0b1d297a58dfe3857f5e5287762aa50dea6036
requirements:
  run:
    - attrs
    - ruamel
test:
  imports:
    - demes
about:
  home: https://github.com/popsim-consortium/demes-python
  PyPI: https://pypi.org/project/demes
  summary: tools for describing demographic models
  license: ISC

package:
  name: demes
  version: 0.2.2
source:
  url: https://files.pythonhosted.org/packages/2f/e4/86796d77ea453aa96f1ce758b461cdf738177f441a18c832cc9f29848903/demes-0.2.2-py3-none-any.whl
  sha256: 54f0110e9f1b5167347fb4793f0b1d297a58dfe3857f5e5287762aa50dea6036
requirements:
  run:
    - attrs
    - ruamel
test:
  imports:
    - demes
about:
  home: https://github.com/popsim-consortium/demes-python
  PyPI: https://pypi.org/project/demes
  summary: tools for describing demographic models
  license: ISC
