[Flags] Selective tag pemission

## Bug Report

**Current Behavior**
If a user has permission to view flags in _some_ tags, but not others, details of a flag within a tag with no permission are still displayed in the flags notification dropdown. Note: I do not have permission to view flags in the example discussion

**Screenshots**

**Environment**
- Flarum version: 1.0.2
- Website URL: http://discuss.flarum.org
- Webserver: [e.g. apache, nginx]
- Hosting environment: [e.g. shared, vps]
- PHP version: x.y.z
- Browser: [e.g. chrome 67, safari 11]

```
Output of "php flarum info", run this in terminal in your Flarum directory.
```

**Possible Solution**
I'll see about putting a PR together ;)

Reproduce the issue in the flags notification dropdown with a user who can view flags in some tags but not others, using the reported selective tag permissions. Trace how the dropdown decides which flag details to display; done means flags from unauthorized tags are not shown while permitted-tag flags remain visible.

Written by the indexing model from the issue text.