Improve design of detail view for one register object

• Make that the buttons don't overlap on small sizes (suggestion: full-width, one row for each button on size s)
• Restructure heading: Make font size smaller, use two lines (first: date + period, second: group, subject, teacher)
• Show explanation for status icon and put into a better fitting position
• Fix tab bar
• Use short names for all items in the heading (subject, group, teacher)
• Remove "back to" from "Back to week overview"
• Use short name for subjects in buttons