Make the layout usable on large screen, instead of having a thin collumn

[TheFrenchGhosty]

A screenshot is better than a thousand word:

Literally more than 2/3 of the screen is useless, and this is "only" a 16/9 monitor

[syeopite]

Post contents are limited to a maximum width of 540px to replicate Tumblr's behavior on their dashboard layout which does the same thing. Its mostly to just ensure that photosets looks exactly the same though.

This can probably be changed

Related: syeopite/npf-renderer/issues/10

[TheFrenchGhosty]

@syeopite Why not just keep the size on images but not the rest? Also... an arbitrary pixel limit instead of a percentage is just weird