Mail: Person search: Limit reactivity

[benbucksch]

• Avoid that the $: reactivity statements trigger on the value that they change. Put the action in a function.
• Avoid re-running the search twice for the same person
• Handle that the user clicked elsewhere before the search completed.

Helps with #368, but not a complete fix

[nix-universe]

With this change, the person list is not rendered in the pane at all. Without the change it is rendered.

[benbucksch]

It works for me. Did you try to restart the app?

[nix-universe]

Locally tested: this resolves the 500,000 mail db request issue.