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)
- 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, closes part of #18897

## Screenshots

### Inline flash message (before)
![before](/uploads/c5b3cc05140eaeb9d14e481fa506ebbf/before.png)
![before-mobile](/uploads/004b1f2b9510bd97f4f8a7a6e56f17ef/before-mobile.png)

### Inline flash message (after)
![inline-flash-message](/uploads/fe2a23b63623612b696d529e81fa459e/inline-flash-message.png)
![inline-flash-mobile](/uploads/6207604acfebcf7402d77d5638c89ca8/inline-flash-mobile.png)

### Other flash messages (after)

![flash-message-issue](/uploads/d7e12bd0da68cfa333471ed102428cec/flash-message-issue.png)
---
![flash-new-project](/uploads/3465967dba83296068f154f149d26f64/flash-new-project.png)
---
![flash-sign-in](/uploads/4a5b317d5a369034f43343031607a0c2/flash-sign-in.png)
---
![flash-file-comment](/uploads/488a1119374be7e2173c0a590cfd8821/flash-file-comment.png)
---
![flash-fork](/uploads/f41964e8b910801e03eef3d7649e5009/flash-fork.png)
---
![flash-update-profile](/uploads/9fd972d6c6609fbbf86afcd99d343b5f/flash-update-profile.png)
---

### This is possible now

![flashs-everywhere](/uploads/07f425534511fb4ecaa1d7e2a9870787/flashs-everywhere.png)


See merge request !4854