Make the metadata panel opening affordance more obvious

- rearrange DOM structure of above-fold part of the metadata panel:
  - rename .mw-mmv-controls to .mw-mmv-above-fold
  - the above-fold part is a single positioned div now, with height
    explitcitly set
  - less LESS gymnastics, above-fold height is a single variable
  - add paddings to the p elements instead of the containers
  - make all title elements align to baseline (except the logo which
    would look horrible)
- discard some CSS which was superfluous
  - overspecified sizes/positions
  - some top/bottoms for staticly positioned elements
- get rid of the .mw-mmv-drag-affordance div, since a full-width bar
  wouldn't really make sense on the bottom of the above-fold section
- flip the chevron and place it to the bottom of the above-fold part;
  add colors etc. per spec
- fix stripe button horizontal spacing

Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/706
Change-Id: Ic37b4150288055c3fae8d22919ed7b1249db1f09
(cherry picked from commit 5e3808a14e83b2a656eeddb41bcd59f192c7baff)