Refine header

There are some changes for the header:

• the arsnova title should be centered
• the back button should have a description of where it goes
• instead of the e-mail should be used an icon which symbolises the role

created branch 189-refine-header to address this issue

marked the checklist item the arsnova title should be centered as completed

marked the checklist item instead of the e-mail should be used an icon which symbolises the role as completed

added WIP label and removed ToDo label

added 1 deleted label and removed WIP label

assigned to @lmas60

marked the checklist item the back button should have a description of where it goes as completed

unassigned @lmas60

assigned to @tksl38

mentioned in merge request !129 (merged)

closed via merge request !129 (merged)

mentioned in commit 59300de6