Improve issue forms

[CamillaLu]

This pull request is the start of improvements to the issue forms. So far, I have made the following changes:

• Moved the problem description below the environment section to clarify that the form contains multiple fields even on small screens
• Made the problem description required
• Fixed the code to reproduce rendering using the "python" class (I also tried "code" but it does not provide much syntax highlighting)

The following issues are still open:

• The drop-downs with possible Python and pyGIMLi versions need to be kept up to date, using an input field with regular expression validation would be better
• Making OS and Python version required but only if the output of pygimli.Report() is not provided

Unfortunately, it is currently not possible to do this with the issue form validation.

The forms can be tested here.