Change block "label" so thad a title like everything else in the template file (and all other template files)

Problem/Motivation

Look at this code, it's an inconsistent mess of title_prefix, title_attributes and label!

<div{{ attributes }}>
  {{ title_prefix }}
  {% if label %}
    <h2{{ title_attributes }}>{{ label }}</h2>
  {% endif %}
  {{ title_suffix }}

  <div{{ content_attributes }}>
    {{ content }}
  </div>
</div>

Proposed resolution

The fix: The variables printed in a template file for a title of a block should to be called title.

User interface changes

- none

API changes

- none

Related Issues

#1591806: Change block "subject" so that it's called a (admin_)label like everything else on the theme layer
#1591830: Change comment "subject" so that it's called a title like everything else in the template file (and all other template files)
#1825216: Name variables consistently across all templates (preprocess)
#1939234: Change node "label" so that it's called a title like everything else in the template file (and all other template files)

Beta phase evaluation