Make MAGMa web python3 compatible

[ridderl]

All work on python3 compatibility was done in branch "py3" so far.

MAGMa job works on py3, but MAGMa web still fails.
I got all the tests running, but then running a job from the web interface still failed.
2 issues I couldn't fix:

• Writing the msdata file: read/write as text or as bytes?
• No script.sh was written when submitting a job from the interface

[ridderl]

The tests for magmaweb pass now, but the user interface still doesn't work properly with python3. (With python2 it works fine.) The problem when running with python3 is now also that the validation of the input form for ms data fails. Click one of the examples, select a compound database (e.g. HMDB) and submit: now it complains that no MS file is specified. That should not happen since MS data is given in the text box.