Prompt template field UX improvements

Two suggested improvements for prompt templates:

1) when tabbing between {{template_variables}}, the textarea should scroll to the current cursor. Currently, when tabbing between template variables, if the next one is out of the visible area, the cursor just disappears.

2) extracting the template variables into a lightweight form to fill them out when chatting would both solve the above problem, and also neatly handle the use case where a variable is used multiple times in the prompt: currently, a user will have to type all 2..n instances of the same variable one by one.