Remove pinTo from Flash

What does this MR do?

• replace pinTo of Flash by parent parameter in constructor
• move positioning of layout containers from CoffeeScript to layout files
• adjust styling of inline flash messages

Are there points in the code the reviewer needs to double check?

• display of flash messages in general

Why was this MR needed?

• allow finding and positioning flash message containers within layout files
• allow adding different CSS classes to flash containers within layout files (necessary for fixing #18908 (closed))
• allow multiple flash messages to be displayed at different places at the same time
• make inline flash messages look nicer

What are the relevant issue numbers?

closes #18908 (closed), closes part of #18897 (closed)

Screenshots

Inline flash message (before)

Inline flash message (after)

Other flash messages (after)

This is possible now