Make ZeroNet to attempt download the un-seeded site in periodical intervals

### Step 1: Please describe your environment

* ZeroNet version: 0.6.4 rev3729

### Step 2: Describe the idea:

#### Steps to reproduce:
Hello, regularly happen that new site is advertised, but you can not access it because of error "content.json download failed" (possibly seeder went offline too early)

For the sake of good sites propagation, it can be handy that once there is "content.json download failed" error, it will also show a button: "Try to download this page in background".
Once button is clicked it will show a bubble for a few seconds saying:
"Zeronet will attempt to download this page in background each 10 minutes for next 7 days.
Once page is downloaded, you will be notified via ZeroHello newsfeed.
You can close this window."

Another approach (not that good and effective in my opinion) can be that once web browser tab is open zeronet will attempt to download unseeded site lets say every 2 minutes. This would remove the need to use browser extension like "Tab auto refresh".

Start from the “content.json download failed” flow and the browser page handling described in the issue. Determine how background retries, the 10-minute interval, seven-day limit, completion notification, and retry controls should fit together; the work is done when the behavior and user feedback are implemented and verified.

索引モデルが issue の本文から書いたものです。