Update logviewer log level colors

[bjarnef]

Prerequisites

• I have added steps to test this contribution in the description below

Description

This PR updates a few things in log viewer:

• Replace anchors with <button> elements
• Use <umb-icon> and fixes issue with toggle arrow up/down icon when open/close "saved searches"
• Fixes issue with self-closing angular <umb-checkbox> component in log levels dropdown (in this case the self-closing custom angular element didn't cause issues except additional whitespace)
• Updates log levels colors to better match the log level type
• Empty state message isn't showm while loading (e.g. when selecting the different log levels from the dropdown)

I think the colors of the log levels was a bit off. Often "orange" is associated with "warning" and "red" is associated with "error".
I have then chosen a dark gray (close to black for "Fatal" log level). Furthermore I have updated "Debug" to use a blue "info" color, since "Debug" and "Verbose" colors is a bit too close to each other.

I searched a bit for "log level colors" and something like these examples seemed to be familiar:

[bjarnef]

Before

After

[nathanwoulfe]

For what it's worth, I like it. 🎨

[nul800sebastiaan]

Looks great to me too! I added rel="noreferrer" to links that have target="_blank" for security reasons.

[bjarnef]

Thanks @nul800sebastiaan
I will try to remember adding this in future for other links with target="_blank"
https://web.dev/external-anchors-use-rel-noopener/