Appearance Themes: if a user has no switch_themes privileges, show the detailed view of the current theme

[matveb]

Instead of showing a mostly empty screen, take advantage of the detailed view each theme has and render that instead.

[markjaquith]

25962.2.diff​ works well for me, and gets my vote for commit.

[nacin]

In 26652:

Themes: When only one theme is available, show just the details view.

Only one theme may be available when no other themes are allowed (multisite), no other themes are installed, or when the user is denied the switch_themes capability but still has edit_theme_options.

props matveb, markjaquith.
see #25962.

[nacin]

Two things left after this commit:

• Pressing the escape key causes the details view to vanish. Wonder what else is possible here. Arrow keys don't seem to do anything, so that's good.
• matveb was going to tweak the responsive breakpoints, I think.

[matveb]

Disables collapse actions when in single-theme mode, improve the responsive layout

[nacin]

In 26660:

Themes: Disable collapse actions when showing only a single theme; improve the responsive layout.

props matveb.
fixes #25962.