Implement roll out/unroll animation for sidebar sections

[alya]

We should implement animations for collapsing/expanding sidebar sections, as in https://terpimost.github.io/zulip-sidebar/.

[zulipbot]

Hello @zulip/design, @zulip/server-sidebars members, this issue was labeled with the "area: right-sidebar", "redesign" labels, so you may want to check it out!

[rahulharpal1603]

@alya Hi! can I take up this issue?

[laurynmm]

@rahulharpal1603 - See our contributing guide with detailed instructions on how to find an issue to work on: https://zulip.readthedocs.io/en/latest/contributing/contributing.html

[gurramkarthiknetha]

Could you please assign this issue to me? It was my first contribution, and I would like to continue contributing to this organization.