WIP: bugfix: Use server_timestamp for presences

[neiljp]

What does this PR do, and why?

We previously relied on local time for determining presence, rather than the returned timestamp.

Outstanding aspect(s)

• [ ] Tests
• [ ] Validate assumptions
• [ ] Potentially split out refactoring
• [ ] Determine aggregate or not, and future of presence
• [ ] Improve presence waiting

External discussion & connections

• [x] Discussed in #zulip-terminal in Users list not showing all the online users #T527
• [ ] Fully fixes #
• [x] Partially fixes issue #527
• [ ] Builds upon previous unmerged work in PR #
• [ ] Is a follow-up to work in PR #
• [ ] Requires merge of PR #
• [ ] Merge will enable work on #

How did you test this?

• [x] Manually - Behavioral changes
• [x] Manually - Visual changes
• [ ] Adapting existing automated tests
• [ ] Adding automated tests for new behavior (or missing tests)
• [ ] Existing automated tests should already cover this (only a refactor of tested code)

Self-review checklist for each commit

• [ ] It is a minimal coherent idea
• [ ] It has a commit summary following the documented style (title & body)
• [ ] It has a commit summary describing the motivation and reasoning for the change
• [ ] It individually passes linting and tests
• [ ] It contains test additions for any new behavior
• [ ] It flows clearly from a previous branch commit, and/or prepares for the next commit