Refactor template editable change state tracking

### Description

Right now, the change state tracking for a template happens inside the own template object (via this.editable.changed).
Except Vue is configured to watch for changes on `this.editable` and uses that to set `this.editable.changed` i.e. the watch handler is mutating the object it is watching.

https://github.com/FlowFuse/flowfuse/blob/af3ed3af4da648bf1cb418aef728c250a463be5b/frontend/src/pages/admin/Template/index.vue#L103-L113

(Note this.editable is being mutated inside the watch handler)

We should separate out the field change state and template properties.

Related issue https://github.com/FlowFuse/flowfuse/pull/3012

### Epic/Story

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### Have you provided an initial effort estimate for this issue?

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