This PR introduces a basic AdminNotice model to store these notices. Admin notices are categorized by their source/type (currently only notices from problem check.) They also have a priority.
This commit switches `DiscourseIpInfo.mmdb_download` to use the
permalinks supplied by MaxMind to download the MaxMind databases as
specified in
https://dev.maxmind.com/geoip/updating-databases#directly-downloading-databases
which states:
```
To directly download databases, follow these steps:
1. In the "Download Links" column, click "Get Permalink(s)" for the desired database.
2. Copy the permalink(s) provided in the modal window.
3. Provide your account ID and your license key using Basic Authentication to authenticate.
```
Previously we are downloading from `https://download.maxmind.com/app/geoip_download` but this is not
documented anyway on MaxMind's docs so this URL can in theory break
in the future without warning. Therefore, we are taking a proactive
approach to download the databases from MaxMind the recommended way
instead of relying on a hidden URL. This old way of downloading the
databases with only a license key will be deprecated in 3.3 and be
removed in 3.4.
