Add a default network theme option

[Mista-Flo]

This new ticket is here to summarize the old ticket #13743.

The goal

Let network administrators choose a network default theme.

Technical Approach

• UX : Network > Themes list table with a row action "Set default theme" (this will be a network option)
• Add an indicator on the same page to say "This theme is the default theme", like super admin in user list page.
• Add a restriction that the theme cannot be deleted
• Keep Backward Compatibility with WP_DEFAULT_THEME constant, that means, if the constant is set, we can't let user choose another default theme : How handle this behavior ?

[Mista-Flo]

#13743 was marked as a duplicate.

[jeremyfelt]

@Mista-Flo Thanks for consolidating the info. Let's keep using #13743 for this as it's nice to have all of the conversation in one place. It's okay that the direction changes part way through. :)

Can you leave a comment there with the information that you have summed up in the description for this ticket?