Fix Bug 1071591 - Download description alignments

To keep headings aligned at the top I removed vertical-align:centre and to keep buttons aligned at the bottom I absolutely position them at the bottom in a space created by adding padding to the bottom of the description.

Also added a fix to keep the "headings" on a separate line on mobile since they look weird on the same line as the image.
The height of the button is linked to the height of the font inside the button, if the user has increased their default font size, the amount of space we're reserving for the button needs to increase as well.