Fix column size for 1280px wide displays

Open Dominik V. Salonen requested to merge quad/mastofe:patch-1 into pleroma

I have no idea if this is the correct location for this merge request, if not, please feel free to tell me.

1280px is a very common screen width (1280x720, 1280x800, 1280x1024, 2560x1440 at 2x HiDPI = 1280x720). Yet Mastofe's minimum column size is just barely too high for this common resolution, introducing pointless "wiggle" and wasting a couple bottom pixels for a scrollbar.

This slight fix doesn't seem to introduce any issues either.

Current mastofe pre-fix: Screenshot_from_2019-04-17_16-30-34

Mastofe post-fix: Screenshot_from_2019-04-17_16-31-05

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