Warn before closing a form with unsaved changes

[taylordowns2000]

Starting with the job builder because it's the most high-impact, we need an elegant and generic way to detect when a form has unsaved changes (maybe through the validate hook?) then then display a warning before the user closes.

This is particularly important on the job builder since clicking anywhere else on the canvas will close the form. See @josephjclark 's old #280

• When making changes to a job, a user's changes aren't saved until the "save" button is pressed (this itself is arguably a little bit anachronistic).
• If the user refreshes the page, navigates away or goes "back" (easily done with a misplaced gesture), all changes will be lost without warning.
• This is particularly painful when writing job code.

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• Use click event on close/cancel to check changeset if changes exist
• If exists, call client event to do https://hexdocs.pm/phoenix_live_view/Phoenix.Component.html#link/1

[NickOpenFn]

Do in the next sprint, requires UI

[NickOpenFn]

Noting that this is badly spec'd for taking into the sprint

[elias-ba]

Technically blocked. See this question on the Elixir Forum.

[NickOpenFn]

Moving into backlog. When planned again will need a spike to look at alternatives.

[taylordowns2000]

@stuartc , this also relates to #2007 ?