Forums: Match contributors by user_nicename if matching by username failed

[SergeyBiryukov]

Background: ​Slack discussion.

@uriahs-victor is listed as a contributor on ​https://wordpress.org/plugins/tweet-old-post/, but does not get a Plugin Contributor badge in plugin support topics: ​https://wordpress.org/support/topic/plug-in-error-9/.

It's because the plugin's readme.txt lists user_nicename (uriahs-victor) instead of user_login (Uriahs Victor).

We don't display the actual username on WP.org profiles though. On ​https://profiles.wordpress.org/uriahs-victor, the heading is a display name, and @uriahs-victor on WordPress.org is a user_nicename, which is misleading, as both are not what we expect in readme.txt. At the moment, the only place that displays the actual username is the old bbPress 1.x forums profile: ​https://wordpress.org/support/profile/uriahs-victor.

To make this more friendly and less confusing, we should match plugin contributors by user_nicename in addition to user_login. The new plugin directory appears to handle this correctly​, it's just Directory_Compat::get_authors()​ and ::get_contributors()​ that need to be updated.

[coffee2code]

In 5287:

Support Forums, User Badges: Use get_authors() and get_contributors() from \WordPressdotorg\Forums\(Plugin|Theme) instance rather than duplicating their functionality.

See #2697, #2701.

[coffee2code]

In 5288:

Support Forums, User Badges: Use user_nicename instead of user_login when checking for user being an author or contributor.

See #2701.
Fixes #2697.