[META] Create an MVP for adding and re-using Media

Problem/Motivation

For Drupal 8.6, we want to ship with a media library that:

1. Provides a nice visual interface for browsing through all the media items in your site
2. Can be opened from a node form (or similar author-facing context) in order to visually select media items to reference in a field
3. Provides a way to upload several media assets into Drupal, quickly enter metadata for them, and reference them in a field -- all without having to leave the page you're on

Proposed resolution

Implement an MVP media library, based on the user stories and design from #2796001: [prototype] Create design for a Media Library.

This was discussed at DrupalCon Nashville, and we have come up with a plan to land the functionality in three issues, each of which is targeted for 8.6 and blocks the one after it:

1. A Media Library view and associated display mode, with no integration with the Field API, and no dependencies on other issues. This will keep the patch reasonably small, and let reviewers focus on styling and usability. This will essentially be a re-roll of #60, and is happening in #2962110: Add the Media Library module to Drupal core.
2. A field widget for re-using media items from a node form (or other authoring context). This will be done in #2962525: [PP-1] Create a field widget for the Media library module.
3. An addition to the field widget to create new media, probably based on the patches from #2938116: [PP-2] Add a bulk upload form for media. The work will happen in that issue.

All of this will be implemented in an experimental module, called Media Library. Ideally we will be able to merge this module into the main Media module before 8.6. If we manage to implement all three of the issues above on time, we can merge Media Library into Media before 8.6 reaches beta.

User interface changes

This is a new UI, so no existing user interfaces will be affected by this change.

API changes

We may be implementing new APIs for implementing Views-based entity selection, but they will live in the experimental Media Library module. At some point in the future we may decide to port these APIs into Views, but that's not currently part of the scope.

Data model changes

None.