Issue 1491: Add default values for name (and alt text for images) in add media form

[mjordan]

Note: Do not merge. This PR has been replaced by #783 (same code changes, but I deleted the source repo since opening this PR).

Github Issue: Islandora/documentation#1491

What does this Pull Request do?

Provides default values for the media name field, and for Image media, a default value for the Alt text field.

What's new?

Implementation of hook_form_alter() to populate the media name field, and an implementation of hook_page_attachments() + the addition of a Javascript file to populate the image Alt text field.

How should this be tested?

1. Check out this branch.
2. Clear your cache (e.g. drush cr)
3. Create a new Islandora object with the model "image" and add an image media. The image Name and Alt text fields should be automatically populated with the parent node's title.
4. Create a new Islandora object with a model different than "image" and add a media. The media's Name field should be automatically populated with the parent node's title.

Additional Notes:

• Does this change the interface, add a new feature, or otherwise change behaviours that would require updating documentation? Yes, improves the UX by providing sensible default values in form fields.
• Does this change add any new dependencies? No.
• Does this change require any other modifications to be made to the repository (ie. Regeneration activity, etc.)? No
• Could this change impact execution of existing code? Yes.

Interested parties

@Islandora/8-x-committers