Add page title to environment_indicator entity forms

Problem/Motivation

The environment_indicator entity forms don't have form titles, so when you view the form in the Seven theme, you just see the shortcut star button.

Proposed resolution

Add form titles to the add/edit entity form pages, like other entity forms in Drupal core.

Remaining tasks

Redefine the environment_indicator.default form handler as separate add/edit handlers, so we can detect the operation in the form builder method, and use a different title for each.
Set the title of the edit form dynamically, with the entity label.

User interface changes

The admin pages to add/edit an environment get proper titles.

API changes

The environment_indicator config entity type gets separate form handler names, but they still use the same EntityForm class.

Data model changes

None.