Change chat message timestamps to 12-hour or 24-hour time

[JasonWeill]

Problem

Relative timestamps take up substantial space and, especially when a message was sent less than a minute ago, can be distracting with their frequent updates.

Proposed Solution

Use HH:mm format, in either 12-hour or 24-hour time, for timestamps in chat messages.

Additional context

We may be able to use the browser's locale for time formatting.