[WS-3120] feat: add multiselect functionality to InputSelect

[srwang]

Description

This PR implements multiselect functionality on the InputSelect component, which is currently being used widely across Publisher.

It looks like InputSelect will be deprecated, so at some point Publisher will need to be updated to use the newer Paragon Form components, and multi-select functionality will have to be implemented there. However, I thought that might be a separate scope of work - this PR is just to get the multiselect functionality into Publisher in the short-term so that we can use it.

See https://2u-internal.atlassian.net/browse/WS-3120

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[adamstankiewicz]

@srwang I'm inclined to close this stale PR for now given that there's other ongoing work related to multiselect, for example: https://github.com/openedx/paragon/issues/1527 and https://github.com/openedx/paragon/issues/1420. Feel free to re-open or let me know if you feel otherwise, though :) Thanks!