"Own Category" as "Not Sure/Other" entry in category drop-down, fix category...

As reported in #263, the category field completely disappears when using the clear button on the category drop down. My understanding is, that this happened because at that point, the category would be null and stuff breaks. Unfortunatly this wasn't caught based on the types and somehow works differently when debugging the application locally. This should no longer happen.

With this MR, the "Own Category" button is replaced with a entry in the drop down (on first position) representing "Not Sure/Other". If this value is selected, the field for entering a custom category appears, along with the "help text" for custom categories.

The category drop down still starts with no value selected, but clearing it will cause "Not Sure/Other" to be selected. Hints will appear when either the drop down or the custom category text field are empty. The submit button will be enabled if there's a non "Not Sure/Other" value selected or a custom category has been entered.