Give chat timestamp a fix width to fix message alignment issues

[astorije]

Fixes #1581.

I gave 5px headroom to the timestamp width I get on my laptop. Tested with/without seconds, on Chrome/Safari/Firefox on Mac, Chrome on Android. Would be nice if someone on Windows and/or Linux could test, just to be sure.

Before	After

[dgw]

Yay for fixing this! But is there a way to fix the width relative to the font size? I can't get in and play around right now, however it seems like something around 5-6em would do unless I'm forgetting something important about CSS sizing…

[astorije]

That could work. What do you think, @xPaw?

[astorije]

@dgw, after chatting with @xPaw, we decided to go for pixels as a quick fix for #1581 because we're not 100% sure how that would behave under different fonts, sizes, etc.

That being said, if you or anyone wants to give it a try, we'd totally consider it in the next release while keeping the quickfix for this release.
I'm merging that now to be able to release v2.5.0-rc.2 right now, but feel free to object and we could totally discuss it before v2.5.0 happens if that seems unwise to you!

[dgw]

Eh, I don't think there's anything wrong with pixels unless font sizes are going to change a lot. This isn't something that would be worth holding back v2.5.0 stable for. If I (or anyone) get around to playing with it in time, fine; if not, also fine!