Space current used by optional files shows space even for 0% downloaded files

### Step 1: Please describe your environment

* ZeroNet version: 0.6.2 rev3467
* Operating system: Linux 64bit Centos7
* Web browser: Firefox beta
* Tor status: error
* Opened port: opened

### Step 2: Describe the problem:

On the Files tab of the ZeroHello i see: "Used: 10.0 GB / 7.5 GB (119%)" and when i ticked & deleted some large files with status 0% (assume these are not downloaded and these are ones that are larger than the maximum limit i set when "Download and distribute all files" was ticked in zite (0) menu) then i refresh ZeroHello Files page and see "Used: 9.0 GB / 7.5 GB". So it decreased. Maybe ZeroHello should not count not downloaded files into limit as it is confusing?

Start at the ZeroHello Files tab and trace how the displayed space usage is calculated for optional files. Reproduce the report with optional files at 0% downloaded, then verify that the usage total no longer includes those files while downloaded files remain counted.

索引モデルが issue の本文から書いたものです。