ui: prune edit: prepare for being re-used for backup jobs

ui: prune edit: prepare for being re-used for backup jobs

1. Rename the isPBS variable to match the actual purpose. When editing
a backup job, the hint should not be shown, even if the storage is
PBS.

2. Allow specifying the text used for the fallback hint. For backup
jobs, the first fallback is the storage configuration, so the text
needs to be different there.

3. Make using keep-all=1 by default upon creation optional. For backup
jobs, the default should be "use fallback", i.e. no settings.

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>