Metatag uses plugin attributes to generate output to simplify output logic

As part of a codebase refactoring, the attributes on meta tag output was extended to use additional attributes which define elements of the HTML tag itself, rather than relying on overriding the output() method. This reduces the amount of output() methods.

Modules that provide additional meta tag plugins can continue to use their existing output() methods, but they may wish to change to using the new plugin attributes instead.

Along with this the following changes have been made:

• Drupal\metatag\Plugin\metatag\Tag\MetaNameBase::$nameAttribute is now deprecated and will be removed in 2.0.0; MetaNameBase::$htmlTagNameAttribute should be used instead.
• theme_metatag_mobile_android_app() and theme_metatag_mobile_ios_app() are deprecated and no longer actually used, instead the meta tag plugins use the improved output logic; these functions will be removed in 2.0.0.