Ext Mgr Version Check Should Respect Branches

The version check feature of the extension manager is not branch-aware.

This means, a user of phpBB 3.1.x would be notified an extension is out of date and needs updating, if that extension happens to have a new release just for phpBB 3.2.x, while in fact the 3.1.x release may be current and up to date.

This could cause some users to install incompatible extensions on their 3.1.x boards. At the very least it will also prevent users from knowing when their extensions truly are out of date.

The extension manager should be able to dicern the difference so it can tell users when the extension from the branch they are using is the one that's been updated.

Version info files from titania will contain branches if the extension has separate downloads/revisions for the 3.1.x line and 3.2.x line. For example:

{

    "stable": {

        "1.0": {

            "current": "1.0.3",

            "download": "https://www.phpbb.com/customise/db/download/xxx",

            "announcement": "https://www.phpbb.com/customise/db/extension/xxx/",

            "eol": null,

            "security": false

        },

        "2.0": {

            "current": "2.0.0",

            "download": "https://www.phpbb.com/customise/db/download/xxxx",

            "announcement": "https://www.phpbb.com/customise/db/extension/xxx/",

            "eol": null,

            "security": false

        }

    }

}