Add "Users" page

[PIG208]

Previous issues from the legacy mobile app repo:

• Create a "Users" panel zulip-mobile#5495, initial issue for this feature
• UI redesign: users screen zulip-mobile#5512, the previous redesign proposal

The feature was also brought up in #119 (comment), which is from the issue for recent DM conversations. Before we can start working on this, we should revisit the redesign proposal first and see if it still does everything we need.

CZO discussion.

[chrisbobbe]

This came up in beta feedback:

Please add online user list to make it easier to DM other users. At this moment there is no way to DM a user who is not active in channels.

Alternatively a list of users who were active in a topic or channel. Useful on really active realms.