MergerSite - Zite retains old merger_type permission after modifying wrapperPermissionAdd.

### Step 1: Please describe your environment

* ZeroNet version: 0.7.2
* Operating system: Manjaro
* Web browser: Firefox
* Tor status: disabled
* Opened port: yes
* Special configuration: no

### Step 2: Describe the problem:

I cloned GitCenter.
I downloaded GitCenter and then rewrote it to my liking in order to start my own GitCenter, called ZeroSafe.
My site however is still looking for GitCenter sites.
I've noticed that this is due to this function:

```
zeroPage.cmd("wrapperPermissionAdd", ["Merger:ZeroSafe"])
```

This adds a permission, but does not set it.
So whenever I change it, it adds another permission
and when I went a little digging I could not find any command that removes or empties the permissions.
I can't manually remove the in sites.json either and I have no idea where these permissions are stored.

#### Steps to reproduce:

1. Open a couple of Gitcenter projects.
2. Recreate Gitcenter, name it Newcenter.
3. Edit the following in your Newcenter site:
```diff
-- zeroPage.cmd("wrapperPermissionAdd", ["Merger:GitCenter"])
++ zeroPage.cmd("wrapperPermissionAdd", ["Merger:NewCenter"])
```

#### Observed Results:

error
```plain
Internal error: Exception('Merger site (NewCenter) does not have permission for merged site:
18r929Hq8hE5Pi9Zf7ovMQVyn3WcGfxMgu (GitCenter)'): Merger site (NewCenter) does not have permission for merged site:
18r929Hq8hE5Pi9Zf7ovMQVyn3WcGfxMgu (GitCenter)
UiWebsocket.py line 80 > 238 > MergerSite/MergerSitePlugin.py line 168 > 136 > 43
```

sites.json
```
permissions: { ..."Gitcenter", "NewCenter"...
}
```

#### Expected Results:

Absention of GitCenter permissions.

MergerSite/MergerSitePlugin.py の 168 行目付近と、UiWebsocket.py の 80 行目付近にある呼び出し経路から始めます。wrapperPermissionAdd を使って権限変更を再現し、その後、merger の権限がどのように保存・更新されるかを追跡します。完了条件は、wrapper の権限を変更しても古い merger の権限が残らず、報告されている GitCenter の検索エラーが発生しなくなることです。

索引モデルが issue の本文から書いたものです。