[views] new cubicweb logo (closes #3803681)
- add the new CubicWeb logo
- use it as a css (background-image)
- change the color of the header bar (gradient made with css)
option_group_changed('port', 'main', 'web')
option_group_changed('query-log-file', 'main', 'web')
option_group_changed('profile', 'main', 'web')
option_group_changed('max-post-length', 'main', 'web')