Align flash messages with left side of page content

## What does this MR do?

- align flash messages with left side of page content

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

- flash messages in general

## Why was this MR needed?

- flash messages didn't fit new design

## What are the relevant issue numbers?

closes  #18688

## Screenshots

# Before

![failed-login](/uploads/fb57d288fb7ae775cc0b8dbb0c7b6af2/failed-login.png)

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![explore-blocked-account](/uploads/aa6d5ff74a4c9ed29ed361f2ffdf5c8e/explore-blocked-account.png)

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![project-blocked-account](/uploads/4f36118f5438d661ea202b180bf9cb8d/project-blocked-account.png)

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![issue-blocked-account](/uploads/68e87385ec30952797dc5c1f8f36216d/issue-blocked-account.png)

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![inline-flash](/uploads/c0aba047f90d56001265e37ba9751e19/inline-flash.png)

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# After

![failed-login](/uploads/d45d2d832a16af8e1854f1832e7b87a4/failed-login.png)

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![blocked-account](/uploads/ddcd3d23569ded403700c01934df61df/blocked-account.png)

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![project-blocked-account](/uploads/9e286c3b95f70cae1819930a09cc1307/project-blocked-account.png)

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![issue-blocked-account](/uploads/4fde8e770eab0dae03b25e1402f171a1/issue-blocked-account.png)

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The left padding will be fixed by  !4854

![inline-flash](/uploads/081b824b99d8ca65d67e9d15778a991d/inline-flash.png)

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![flash-dismissed](/uploads/641e6227050de22e0df9a85ffca4ced1/flash-dismissed.png)

See merge request !4959