Configure and parameterize secondary destination modes

Secondary destinations for backups can be configured with different triggering strategies:

1. Inline (current setting when setting up a backup through the UI), where synchronization is triggered on every succesful backup.
2. Counting, which triggers synchronization every `N` succesful backups (where `N` is configurable).
3. Schedule, which triggers synchronization whenever the last succesful backup was longer than the configured timespan ago.

The mode and parameterization of it are both given as part of the destination in the json, e.g.:

```
{
"destinations": [
{
"url": "s3://bucket/backup",
"mode": "scheduled",
"schedule": "7.00:00:00",
}
]
}
```

Would configure synchronization to happen every 7 days.

The issue describes secondary-destination configuration in the Angular client and the destination JSON shape. Start by tracing the existing UI path for configuring backup destinations and how it emits that JSON. Done means inline, counting with configurable N, and scheduled synchronization with a configurable timespan are represented and submitted consistently.

Written by the indexing model from the issue text.