Upload file - Material gets the ID as name

[jbenito]

Steps to reproduce it:

1. Go to any material widget. E.g.: ​https://indico.cern.ch/conferenceModification.py/materialsShow?confId=XXXXX
2. Add a file or link to a material type that it is not in the default list (so, a custom one). Give the material type any name, for instance, let's call it 'foo'.
3. Delete the resource (file or link). Important: DO NOT delete the material type, only the resource. Of course, the material type will disappear too because there was only one file inside.
4. Now, add a new file or link and choose from the material type list, the type 'foo'.
5. Check out the result: the new file or link was added to a material type that it is not called 'foo' but instead it's got as name the ID of the material type 'foo'.

[Jakub Trzaskoma <jakub.piotr.trzaskoma@…>]

In f8ad7e565ed0c53563a51520bbdbf7eb5394feac/indico:

[FIX] Material type name gets the ID as name

• update material type list when deleting resources
• closes #1094