Sanitize thumbnails and captions styles

Sanitize thumbnails and captions styles

No design changes, just cleaning up.

* Remove dead rules (equivalent to default styles).
* Remove unnecessary !importants (added in r3300 (!), apparently to
  intentionally override styles in skins at the time).
* Make commonContent.css's directionality-specific styling not depend
  on shared.css (these don't have to be loaded together).
* Fix double margins for .magnify in mixed directionalities.
* Clarify that shared.css should only contain basic layout styles.
* Group together directionality-specific styles in commonContent.css,
  comment about them. oldshared.css seems to intentionally not use them.
* Align commonContent.css and oldshared.css where applicable.

Change-Id: I2176d5ee73e37030c15ee5e16ba4eb0660349e15