Redesign: timeline: timestamp placement

[bwindels]

the first message in a group has the timestamp after the sender name; subsequent messages in a group and /me actions show the timestamp after the message, but only on hover.

[bwindels]

Make sure we fix #2284 in the process

[turt2live]

I'm going to close this in favour of modern tracking issues if/when we get them. If people have suggestions for modernized UX, please open new issues for consideration.