PHP Code Review

[tfrommen]

This PR targets PHP-specific code improvements or changes to be made that have been mentioned in #38.

In more detail, these include:

• Import all used classes from global namespace.
• Remove leading backslash for imported global classes.
• Import enqueue function from Asset Loader.
• Use array_diff instead of array_filter.
• Use query API functions instead of WP_Query.
• Replace incorrectly used array_map calls.
• Add constant for CURIE template.
• Refactor filter logic for edit post type views, and prevent the "Mine" view from missing if there is no "All" view.
• Introduce new get_author_ids() function.

This PR does not conflict with any of the other three ones. However, if you decide to not or only partially merge the tooling improvements in #39, you might have to adapt some PHP code, particularly use statements and/or class references in comments. This is also the reason why CI currently fails.

[johnbillion]

This all looks good. The Mine link handling might need reworking again when I get to #15.