[Bug] Error Validating when Primary source type is other

When we tried to validate an entry for which the primary source type is other, for instance, Newspaper, we get the following error:

```python
ValueError: 'Newspaper' is not in list
Traceback:
File "/home/yacine/.local/lib/python3.7/site-packages/streamlit/script_runner.py", line 354, in _run_script
exec(code, module.__dict__)
File "/home/yacine/Code/data_sourcing/sourcing_sprint/app.py", line 380, in
main()
File "/home/yacine/Code/data_sourcing/sourcing_sprint/app.py", line 99, in main
pages[app_mode](submission_info_dict)
File "/home/yacine/Code/data_sourcing/sourcing_sprint/app.py", line 305, in val_page
form_source_category(entry_dict, app_categories, "val")
File "/home/yacine/Code/data_sourcing/sourcing_sprint/catalogue/catalogue_forms.py", line 909, in form_source_category
if mode == "val"
```