Make it possible to pin Direct messages section open

As reported on CZO, some users would find it helpful to always keep the DM section in the left sidebar open. We could add this display option.

We should make this possible to control via a three-dot menu option on the Direct messages row in the left sidebar.

• If not pinned open:
  • Text: Always keep DMs section expanded
  • Icon: zulip-icon-expand
• If pinned open:
  • Text: Collapse DMs section when scrolled down
  • Icon: zulip-icon-collapse

It should also be possible to manage this configuration via a settings option in SETTINGS / PREFERENCES > Navigation (we can put it last).

• Label: Automatically collapse direct messages section in the left sidebar
• Options: When scrolled down [default], Never

We will need to document this feature in the help center, either as part of the initial PR, or as a quick follow-up.

Blockers:

• We should think about it after the left sidebar redesign, which will change some aspects of how sections collapse. We may decided that this feature is not needed at that point.
• Rearranging icons in the Direct messages row in the left sidebar to make room for a 3-dot menu.

Related issues:

• #34470

CZO thread