Improve notifications

[oliver-stoehr]

The various notifications throughout the system are in many cases inconsistent, mislabeled and styled with misleading colors.
Some notifications are shown as growl on the right (sometimes self-closing, sometimes sticky), some are shown as a messages bar spanning the page's full width. Some notifications are displayed multiple times.
Some notifications are displayed in a color, indicating success, while containing a neutral information.

E.g. the colors for warnings and errors are hard to distinguish.

E.g. when starting an export, the confirmation is displayed three times: As full-width-message, growl and sticky growl.
The color indicates success, even if the export immediately fails because of validation (a neutral color would be more suitable to indicate ).

All notifications should be revised to prevent misunderstandings.
The color scheme should be improved to be clearer and easier to distinguish, e.g.:
Red - Error
Yellow - Warning
Green - Success
Blue - Neutral

Implementation estimated to be low.