Editor: Refactor page templates to use Redux state

[aduth]

This pull request seeks to deprecate Flux-based page templates state in favor of global Redux-based state introduced in #7174. It introduces a new <QueryPageTemplates /> query component, refactoring the <EditorPageTemplates /> component to make use of it. Since this is the only area of the application using Flux-based state and the <PageTemplatesData /> component, both of these were removed.

Testing instructions:

Verify that page template options are shown and can be managed, especially with the following considerations:

• Page templates should only be shown when on the page editor, not for other post types
• Page templates should only be shown when two or more options exist
• Page templates should be updated when navigating back to the editor after having changed the theme (see Editor: page templates stuck from previous theme #5387, Editor: Update page templates on theme change #5785, cc @jeremeylduvall)

Test live: https://calypso.live/?branch=update/editor-page-templates

[timmyc]

Nice - This could be used in Tags & Categories 👍

[timmyc]

Testing out well for me 🚢

[gwwar]

👍 The code here looks good and I wasn't able to break this while testing.

[aduth]

Ran into a couple issues during final round of testing:

• Looks like the previous <PageTemplatesData /> component was responsible for injecting the default template option, and this wasn't reflected with the original changes here. Default option should once again be available in the list as of c956700.
• While current flows would not have surfaced this as an issue, page templates should be refetched if the theme changes for a site while the component is mounted. This is reflected in dcbda64.

To the latter point, this reminds me that I'd once considered perhaps keying page templates by theme rather than by site, since this would allow us to reuse the page templates between two sites both using the same theme. Looking into this some more, aside from potential conflicts on theme slug, there's also the issue that themes can be enhanced with more page templates on a per-site basis. For example, a standard WordPress.com site enables an "Eventbrite Events" page template for the Twenty Sixteen theme, which wouldn't be available for a Jetpack site using the same theme.

[timmyc]

Looks like the previous component was responsible for injecting the default template option, and this wasn't reflected with the original changes here.

Good catch - I knew there was a bug I had fixed in this component before and that was it. There are a few themes with only one page template, comics is one for example, and confirmed it is now properly showing the drop down with default and full-width as options 👍

While current flows would not have surfaced this as an issue, page templates should be refetched if the theme changes for a site while the component is mounted

Definitely an edge case, but maybe multiple tabs could be open to arrive at such a global state.

Tested again, still looking good 🚢