Fix heatlegend select

[JonasGilg]

Description

This PR fixes the previews of heatlegends in the heat legend select dialog.

Related Issues

Fixes #326

Design Decisions

Instead of using amCharts to render the heat legends we use a simple CSS gradient.

Checklist

I, the author of this PR checked the following requirements for good software quality:

• [x] The code is properly formatted (I ran the formatter)
• [x] The code is written with our software quality standards (I ran the linter)
• [x] The code is written using our code style
• [ ] Extensive in source documentation has been added
• [ ] Unit and/or integration tests have been added
• [x] All texts have been internationalized with at least the following languages:
  • [x] English
  • [x] German
• [x] I tried addressing all new accessibility problems displayed in the console and documented if they can't be fixed
• [ ] I attached performance measurements to prevent performance degradation
• [x] I added the changes to the next release section of the changelog

I, the reviewer checked the following things:

• [x] I ran the software once and tried all new and related functionality to this PR
• [x] I looked at all new and changed lines of code and commented on possible problems
• [x] I read the added documentation and checked if it is understandable and clear
• [x] I checked the added tests for completeness
• [x] I checked the internationalized strings for spelling errors
• [x] I checked the performance metrics for problems or unexplained degradation
• [x] I checked that the changes are noted in the changelog