Inform user when trying to download where they dont have the full tree locally

[olizilla]

Describe the bug
"Download" gets stuck when you dont have the full tree locally and you are unable to fetch it. Even if you are able to fetch it the download may take a while and appear to stall as the user does not know they are know waiting to find the blocks.

We can check to see if the full tree is available locally before starting the download and warn that we will have to go find the full tree and it could take a while if it isn't all available.

This is easy to trigger know due to the "Import from IPFS" action only importing the root block as per #1691

[jessicaschilling]

From today's triage discussion: Actually implementing a fix shouldn't be too difficult, but need some further investigation first, to wit:

• Determine how expensive it is to tell if an item is available locally or not
• Determine if we (and if so, how expensive is it) can get a percentage of an item that's available locally

If it's expensive:

• Check only at time of download, and if the item isn't fully available locally, give a warning modal that we have to find the full tree and that might take a while if it's not all available

If it's not expensive:

• Add a UI element in the Files screen table to indicate whether an item is fully local or not (perhaps including percentage local availability)

As of time of writing, bandwidth doesn't support our swarming on getting this fixed, but let's keep close to the top of our "quick fixes" priority list.