Enforce width-only default size only on larger images

While the documentation is unclear, it seems that the 'thumbsize' in
$wgDefaultUserOptions is setting up width limit rather than a general
bounding box.

Things that are adapted in this fix:

* Only resize the image if the image is bigger than default and marked
  as default size.
* Only limit width, even if the image is tall.
* Add rounding to calculated height value.
* Make sure new images follow these rules as well on insertion.

Bug: 62293
Bug: 62279
Change-Id: I37525bf3480bb1da8b0538f9b38576616aabbfa6
Merge "Enforce width-only default size only on larger images"