Change title structure on plugin category templates

[jonoaldersonwp]

The <title> on plugin category pages (e.g., ​https://wordpress.org/plugins/category/taxonomy/) is currently {{cat}} | WordPress.org.

This should be changed to Plugins categorised as {{cat}} - WordPress.org

Paginated states (e.g., ​https://wordpress.org/plugins/category/taxonomy/page/2/) should inject the pagination state; e.g., Plugins categorised as {{cat}} - Page {{x}} of {{y}} - WordPress.org

[coffee2code]

In 8579:

Plugin Directory: Include total number of pages in page title for paginated views.

See #4328.

[coffee2code]

In 8580:

Plugin Directory: Change page title format for category archives.

Adds text to provide context about the category.

Props jonoaldersonwp, coffee2code.
Fixes #4328.

[coffee2code]

FYI: The actual implementation differs from the original ticket in two minor ways:

• Uses American English spelling of "categorized" (instead of "categorised")
• Retains the use of vertical bar | as the title parts separator and not a dash -

In #1105 it was decided to use — as the separator between page title parts. However, in #3539, the separator was changed to | (the vertical bar) though I don't see in that ticket or its comments where that particular change was requested or justified (it may be within the private Google Doc that was linked, but I can't see its contents). So right now the vertical bar is the standardized separator. Please open a new ticket if this should be changed since it would be a separate and wider-ranging effort than just this one instance and would be undoing deliberate changes made for #3539.