Allow customization of initial deploy preview interval / maxAttempts

**Is your feature request related to a problem? Please describe.**

GitLab runners take longer to run than the default interval defined here:

https://github.com/netlify/netlify-cms/blob/a843c169a10a1f73e00755a917f127975708a685/packages/netlify-cms-core/src/backend.ts#L991-L1000

Therefore, although deploy preview links are eventually available, the "Check for Preview" button disappears before that. I always have to reload the page first so the interface waits long enough until the preview is available.

**Describe the solution you'd like**

Making interval and/or maxAttempts customizable. I suspect that a reload is always enough for my purposes because the GitLab runner time doesn't exceed the three attempts defined here:

https://github.com/netlify/netlify-cms/blob/f60c2871d30b381fb5f3315c4de77ccdd1650676/packages/netlify-cms-core/src/components/Editor/EditorToolbar.js#L253-L265

**Describe alternatives you've considered**

Always reload after saving changes.