Visual improvements when left sidebar is scrollable

[alya]

Describe the bug

When there are enough organizations in the left sidebar to make it scrollable, there are a couple of things that look awkward:

1. The scroll bar overlaps with the unread counts.
2. The amount of space at the top (above the fist org) and the bottom of the scrollable part (above the bell) seems unnecessarily large.

To Reproduce

Make left sidebar scrollable.

Screenshots

Desktop (please complete the following information):

• Operating System: macOS 11.6.2

• Zulip Desktop Version: 5.8.1