[#892] Tectonic: fix modules in one-column mode small screen

On one-column mode in larger screens, we want the modules to be beside
one another.

On one-column mode in smaller screens, we want the modules to be stacked
on top of one another.

This makes it so

Fixes #892.
Merge pull request #905 from afuna/tectonic-one-column-bug-892

[#892] Tectonic: fix modules in one-column mode small screen