Limit terminal width to parent container size

[oliverroick]

Fixes #67

By setting the min-size of the grid cell to 0, we prevent the children from growing beyond its size.

This resolves the issue described but has size effects:

• If there are breaks in a line there's a tiny scroll bar

• If there are breaks in a line, subsequent output might overwrites existing output.

I couldn't find any xterm settings to change the behaviour so I'm not sure what the best approach is.

[yuvipanda]

@oliverroick I've had to basically use https://xtermjs.org/docs/api/addons/fit/ in the past to deal with any sizing issues with xterm in the past. For example, in https://github.com/jupyterhub/binderhub/blob/a93a01cf620605f20d5874a02ffbf9afd9ef97b9/binderhub/static/js/index.js#L74 (don't judge the rest of the code), we call fit everytime something is written to the terminal, to deal with the second side effect I think.

[batpad]

Oops this PR has been open a while. @oliverroick do we still need to get this over the line to fix #67 ?