KeyError - sidebarGetHtmlTag - SidebarPlugin.py

### Step 1: Please describe your environment

* ZeroNet version: 3853
* Operating system: W10
* Web browser: Firefox
* Tor status: available
* Opened port: no
* Special configuration: multiuser_local

### Step 2: Describe the problem:

Hello, i visited a hub zite several times in last days and its zeronet menu (corner button) shows nothing in the menu, only black and debug.log shows this site relevant entry:

> ERROR Site:abc..def WebSocket handleRequest error: KeyError: u'folder/subfolder /content.json' in UiWebsocket.py line 83 > UiWebsocket.py line 267 > SidebarPlugin.py line 531 > SidebarPlugin.py line 196 > ContentManager.py line 322 > ContentManager.py line 322 > ContentManager.py line 320 > ContentDbDict.py line 61 > ContentDbDict.py line 34
{"cmd":"sidebarGetHtmlTag","params":{},"id":1000002}

(subfolder name contains ":" and there is space at the end. I checked and that folder does not exist on my end.

When i deleted site and opened again, menu started opening. If anyone interested here how i deleted the site while preserving all optional files i downloaded:

> 1. I backed up "folder"
> 2. deleted site
> 3. visited site
> 4. copied back "folder" (and skipped files that existed on newly downloaded site)
> 5. on ZeroHello i "Check files" of that site
> 6. site menu now opens

Start with the sidebarGetHtmlTag request in SidebarPlugin.py, then follow the traceback through UiWebsocket.py and ContentManager.py to the ContentDbDict.py lookup. Reproduce the malformed folder/subfolder content.json case and determine how the missing key reaches the sidebar. Done means the sidebar menu opens without this KeyError for the reported site state.

索引モデルが issue の本文から書いたものです。