Allow to resize the compose textarea

[pabzm]

Sometimes one needs a little bit more space to write an email, then being able to manually draw the edit field bigger is very helpful.

This had been blocked in 1c5f83d, which references #7230. But the problem described in that issue doesn't reappear without this CSS statement (tested in Firefox, Chrome and Vivaldi on Mac OS), so I assume we can safely remove it.

[alecpl]

The textarea (and html editor) grows while you write in it. So, I don't see a reason to resize it manually.

[pabzm]

But there's neither a reason to explicitly block it, is there?

I prefer to see a bigger textarea for longer texts, and having to enter a lot of newlines to achieve that feels like a clumsy workaround.