Add support for Media Library metadata

General:

• This is a progressive enhancement that will live in the System module. Simply check if app::hasDatabase() or something similar before registering event listeners
• Media Manager should have integrations with the Form widget to simply extending it
• Perhaps handle https://github.com/rainlab/translate-plugin/issues/192 as a part of this

Schema:

Table Name: media_library_items
id // To simplify other people's relationships to Media Items
disk // To distinguish between multiple files stored on different disks
path // The path to the media item
metadata // JSON stored metadata.
timestamps?

Model:

Name: MediaItem
Interface:
MediaItem::fromPath($path, $disk = $defaultDisk) // Gets the Media metadata for the specified path
- Provide method to load metadata directly from the identifier passed that would override the stored metadata. I.e. Path could be either a string or an array containing overriding metadata.
- Perhaps have getAttribute() attempt to pull unknown attributes from the ->metadata->$attribute?
- Perhaps have setAttribute() do the same?
- Take inspiration from https://github.com/kodeine/laravel-meta. 

Interactions:

1. Media Metadata integration will listen to the media events and update the DB reference to the path on the appropriate events
2. Attempting to access a MediaItem that doesn't exist in the DB yet will create it in the DB (perhaps only if it exists in the disk?)
3. Perhaps integrate File Interface for using in the file model and Media Item model?
4. Twig interface should be {{ media(path, optionalDisk).mediaItemPropertyOrMethod }}

Perhaps take some extensibility inspirations from https://github.com/ctf0/Laravel-Media-Manager