Implement lovelace-wide zoom option

[igorsantos07]

Is your feature request related to a problem? Please describe.
Some low-resolution devices end up showing only a single column on dashboards, specially tablets. Having the ability to reduce the default zoom and thus, showing more stuff in the screen would help a lot on those scenarios.

This is not related to zooming in pictures or videos temporarily, but zooming in/out the dashboards as set-and-forget setting.

Describe the solution you'd like
As seen here, the iOS companion app has a zoom option which applies to the entire app, or at least to all dashboards (webviews, I guess). I can't confirm this right now as I don't have an iOS device accessible.

A similar option would go a long way for Android, if that's technically possible.

Describe alternatives you've considered, if any
Pinch-to-zoom only allows to zoom in, not out.

Additional context

An option to change the default zoom level was requested in multiple places, and can be easily found on Google:

• https://www.reddit.com/r/homeassistant/comments/1bvvp28/zoom_out_companion_app/
• https://community.home-assistant.io/t/zoom-page-screen-size/269188
• https://community.home-assistant.io/t/zoom-inn-android-app/353758 (which pointed me here to open this FR)
• Implement Zoom feature from iOS into Android Companion app #1481 (got closed for similarity with another issue, which was also active at the time, but the main topic wasn't the same)
• A previous issue about zooming to video (Stream zoom functionality #1117) ended up with:
  • the pinch to zoom PR Add PinchToZoom option #2252 (which solves the main scenario from that issue)
  • some investigation about setting the initial zoom (what I'm talking about here)

[bconte]

Eagerly awaiting this merge! Thank you!