make Template URL always editable

[fiveangle]

Current behavior if one attempts to clear the Template URL field is that the Save button is grayed-out and user cannot clear the field. This can lead to user's inability to resolve constant error in Portainer if the Template URL points to a malformed JSON file. Per #4865 this is apparently expected and desired behavior 🤯

If Portainer is going to disallow the Template URL field being blank, then it should insert the default URL into the field if the user attempts to blank the field, rather than grey-out the Save button and leave the user confused that they can't blank the field or make any changes to any items on the Settings page.

With the current behavior, the user cannot know that the field cannot be blanked and they can't inherently know the default URL so the user experience is indistinguishable from a bug [insert philosophical thought experiment about trees falling in the woods here].

Thanks for your consideration !