Respect a theme's screenshot aspect ratio

Fixes vanillaforums/vanilla/#1894
Prevent super tall screenshots

Use max-height and explicit height: auto.
Merge pull request #1895 from hgtonight/patch-theme-aspect-ratio

Respect a theme's screenshot aspect ratio (within reason)
Thanks @hgtonight!
Default to 25 comments per page
Merge remote-tracking branch 'upstream/2.1' into 2.1