Introduce meta description presenter

[herregroen]

Summary

Introduces the first presenter for indexables. Intended primarily to serve as an example for future development.

This PR can be summarized in the following changelog entry:

• No user-facing changes.

Relevant technical choices:

• Only simple pages are supported for now, in all other cases it falls back to the original behaviour.

Test instructions

This PR can be tested by following these steps:

• Enable the YOAST_SEO_EXPERIMENTAL_PHP56 and YOAST_SEO_INDEXABLES feature flags.
• The meta description on the front-end should remain identical.
• For every post you visit on the front-end a new entry in the indexables table should be created.

Documentation

• I have written documentation for this change.

Quality assurance

• I have tested this code to the best of my abilities
• I have added unittests to verify the code works as intended

[codeclimate]

Avoid excessively long variable names like $post_type_meta_description_presenter. Keep variable name length under 30.

[codeclimate]

Avoid excessively long variable names like $post_type_meta_description_presenter. Keep variable name length under 30.