UX for sharing files that take long time to hit preload servers

> Extracted from https://github.com/ipfs-shipyard/ipfs-share-files/pull/44#pullrequestreview-176787215

### Problem

When sharing big files it takes time for actual data to hit preload servers.

Imagine a scenario:
- You add 3GB file, copy download link, close the tab, share it with friends
- Friends start to download data
- ... but you closed the tab (js-ipfs) or shut down daemon (go-ipfs) before entire thing propagated to preload server
- there is no other source to fetch data from and they are stuck at 15% forever :(

### Solutions

- For files bigger than "X" we probably should display a message warning user that the tab should be open until their friend downloads entire thing.
- https://github.com/ipfs-shipyard/ipfs-share-files/pull/44#issuecomment-440306496
> I agree that it needs to be addressed ASAP, but always and not only when using an embedded `js-ipfs` node. Even if the user is using a local node, if he adds a file, gets the share link and then quits its daemon, the file is "lost". That must be explicit somewhere, and I think it should be somewhere in the copy of "how the share app works".
- ?

Thoughts?

cc @ipfs-shipyard/gui (as this is relevant to all our apps)