Support for custom image width

Adds also beginning of support for custom CSS

Improves the unit tests for ProofreadPagePage and ProofreadIndexPage

Change-Id: I9a6e48c57d97f0bd6683666d680e86c17e93a2ab
Replace jquery.panZoom by a lighter zoom widget

Change-Id: Ief9cf4bdab04a2f5c44510615dfd6e7481a65f06
Merge "Replace jquery.panZoom by a lighter zoom widget" into pagePagesRefactoring
Merge "Support for custom image width" into pagePagesRefactoring