Update Theme Browser to match the latest Web iteration

[kwonye]

The Theme Browser is behind the Web version.

• Update url from wp-admin/customize.php to wordpress.com/customize
• Possibly remove "Try and Customize" in the overflow menu

[kwonye]

This isn't high enough priority for us to get in by 5.6. Moving to 5.7.

[rachelmcr]

The web version also recently launched new theme details pages:

• Update theme Details URL from wordpress.com/themes/THEME_SLUG to wordpress.com/theme/THEME_SLUG
• Update theme Support URL from wordpress.com/themes/THEME_SLUG/support to wordpress.com/theme/THEME_SLUG/support