Configurable scrollback length

The amount of scrollback kept in cache is insufficient for a high-DPI monitor in vertical orientation.

Steps to reproduce:

1. Acquire a nice high-DPI monitor and put it in vertical orientation for Leet Hackerage. (Here's my xrandr output: `DP-2 connected 1440x2560+2560+0 left (normal left inverted right x axis y axis) 610mm x 356mm`) The upshot is that my display is 2560 pixels high.

2. Settings > Appearance > Message Display. Select "Compact". (Note: this is also visible in "Cozy", but "Compact" makes it very obvious.)

3. Open a channel with a high number of short messages (e.g. each message occupies only one line)

Expected:
4. Observe a beautiful scrollback all the way up your miles of monitor real estate, allowing you to view the history of the universe since before the Big Bang.

Actual:
4. Scrollback only occupies about 60% of the screen and there's a lot of ugly vertical space. (https://imgur.com/a/Zn2MO9g)

Note: It's also visible in Cozy mode, although tricker to get to: https://imgur.com/a/3Mc3NmS

It'd be nice if the number of scrollback lines was manually configurable, or was configured dynamically based on the height of the window.

Build:

Stable 54611 (3fb9c49)
Host 0.0.9
Linux 64-Bit (5.2.17-1rodete3-Amd64)

(that's a Debian derivative)