Feature request: monitor the size of blobfolder and downloaded files

Nobody has claimed this yet.

@LavRadis commented on [Thu Mar 15 2018](https://github.com/lbryio/lbry-app/issues/1108)

## The Issue
It would be good to have a size of the blobfiles folder and downloaded files somewhere in the app so the users could monitor for potential problems.
I've experienced an issue that made me aware of the need to keep an eye on those sizes.

### Steps to reproduce
1. I can not reproduce it, but I'm sure the issue will re-appear somewhere in the future
2.
3.

### Expected behaviour
Show the total folder size of the blobfiles folder and total downloaded files sizes.

## System Configuration

- LBRY Daemon version: 0.18.2
- LBRY App version: 0.20.0
- LBRY Installation ID: 7tYN144rHXrspzJG9xLP28fi1JztXnbpKro6DBJEQyzBmXTyDgqRLfYsUvHYFVmMe2
- Operating system: win 10

## Anything Else
I've noticed that I'm completely running out of space on my Win partition (c:) which made my PC slow down, I've decided to check the size of the blobfile folder and discovered it was 8.5 GB. Since I've already deleted all the downloaded videos I tried to manually delete the blobfiles. After deleting them, they started to rapidly re-appear. I've also tried to delete all the remaining files in "downloaded" LBRY app folder, but they also kept re-appearing as I deleted them on the list. At a certain point, after I deleted both blobfiles and downloaded files for several times, they did not return.
I'm not able to reproduce the issue. lol.

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@tzarebczan commented on [Fri Mar 16 2018](https://github.com/lbryio/lbry-app/issues/1108#issuecomment-373800112)

Will most likely require: https://github.com/lbryio/lbry/issues/994

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@LavRadis commented on [Fri Mar 16 2018](https://github.com/lbryio/lbry-app/issues/1108#issuecomment-373802796)

It will certainly need some sort of warnings, monitoring, and user control to set the maximum sizes. When the disc is close to full it already starts to cause problems.

No source files, tests, or entry points are named. Start by reading the linked daemon issue #994 and the originating app issue #1108, then clarify whether the required scope is size reporting only or also warnings and user-configurable limits; done should match the agreed behavior for blobfiles and downloaded files.

Written by the indexing model from the issue text.