UnicodeDecodeError when clicking edit galley

Describe the bug
When an HTML/XML galley file is uploaded with an encoding different than UTF-8, Janeway will throw a server error if a user tries to edit such galley.: UnicodeDecodeError: 'utf-8' codec can't decode byte [...]

Janeway version
v1.3.8

To Reproduce
Steps to reproduce the behavior:

1. Create a non UTF-8 encoded HTML file
2. Upload the file as a galley to any article
3. Hit edit galley
4. See error

Expected behavior
The edit galley page should still resolve properly, perhpas with a message specifying that the encoding of the file is invalid and suggesting the user to upload a properly encoded version of the file.
Additionally we address this problem at upload time, I can think of two approaches:

1. Add a form validator that checks the encoding of the file is supported.
2. Guess the file encoding with a charset detector and transform it into UTF-8 where possible. This however could lead to data-loss or file corruption if we don't guess the encoding correctly.