Themes: turn route paths into reusable router properties

[matveb]

Avoid repeating the same fragment path over and over for the themes.php and theme-install.php routes. Abstracting this into centralized router attributes will also help with porting the themes UX to .org/themes directory.

[ocean90]

What about the routes events?

[obenland]

They can be overwritten by extending the corresponding Router as is.
In the case of the theme repo, it looks like this:

        _.extend( wp.themes.InstallerRouter.prototype, {
                routes: {
                        '/:slug/': 'preview',
                        '/browse/:sort/': 'sort',
                        '/?upload': 'upload',
                        '/search.php?q=:query': 'search',
                        '': 'sort'
                },

                baseUrl: function( url ) {
                        return '/' + url;
                },

                themePath: 'themes/',
                browsePath: 'browse/',
                searchPath: 'search.php?q='
        });

[johnbillion]

In 30508:

Abstract the theme browser route paths into attributes so they can be easily overridden.

Fixes #30116
Props matveb