Remove usage of the `wpurl` blog option

[johnbillion]

There are two instances in core where get_bloginfo( 'wpurl' ) is used, one of which is incorrect and should use admin_url(), and another which should use site_url().

[johnbillion]

34280.diff​ corrects the following:

• Potentially incorrect scheme for the application/rsd+xml link URL.
• Potentially incorrect scheme and/or incorrect URL for the plugin editor error scraping iframe.
• URL generation for the plugin activation error scraping iframe, just for parity with the above.

Just needs a sanity check.

[SergeyBiryukov]

In 35105:

Replace get_bloginfo( 'wpurl' ) with site_url() in rsd_link().

Ensure the correct scheme is used for the application/rsd+xml link URL.

Props johnbillion.
See #34280.

[SergeyBiryukov]

In 35107:

Replace get_bloginfo( 'wpurl' ) with admin_url() in plugin editor.

Use add_query_arg() instead of constructing the URL manually.

Props johnbillion.
See #34280.

[SergeyBiryukov]

In 35109:

Replace get_bloginfo( 'wpurl' ) with admin_url() for plugin activation error message.

Use add_query_arg() instead of constructing the URL manually.

Props johnbillion.
Fixes #34280.