It is possible to overflow the number of items allowed in Media Library

Problem/Motivation

When adding new media through the Media Library, it's possible to add more than the allowed number of media items.

Steps to reproduce:
1) In the Media Library, if you can select two items, select two.
2) Now upload a third item.

Expected: You should not be able to select more than the allowed number, or should receive a warning if you do.
Actual: no warning appears, and if you then insert the media you get nothing is attached. It fails without warning.

Proposed resolution

1. When the number of selected items exceeds the maximum allowed after adding one or more items in the media library, display a warning.
2. If the user then persists on inserting the media items with an overage, do not allow it, but display an error message.

Video:
https://www.drupal.org/files/issues/2019-12-21/3082690-71-walkthrough_0.mov

Remaining tasks

• Discuss solution
• Write patch
• UX Review
• A11Y Review
• Code review
• Frontend framework manager review
• Commit

User interface changes

Added error / warning messages to the media library.

Video of #21: https://www.drupal.org/files/issues/2019-07-17/media-library-exceeds-lim...

The warning after adding and exceeding the limit:
https://www.drupal.org/files/issues/2019-07-17/media-library-add-warning...

The warning after clicking 'Save and select':
https://www.drupal.org/files/issues/2019-07-17/media-library-save-and-se...

The error after clicking 'Save and insert' or 'Insert selected':
https://www.drupal.org/files/issues/2019-07-17/media-library-save-and-in...

API changes

Data model changes

None

Release notes snippet