Allow arbitrary return redirects when activating plugins

[rmccue]

After activating a plugin, the return redirect is set to either import.php or plugins.php depending on the from query parameter. It'd be nice to have the ability to specify an arbitrary redirect_to similar to the login process.

This should probably be locked down using wp_safe_redirect.

[rmccue]

Added initial patch. Needs sanity checking.

[swissspidy]

Had a quick look at the patch. Why does it use wp_redirect() once and once wp_safe_redirect()?

[rmccue]

Whoops, forgot to update that one. Technically, it doesn't need to, as the redirect gets run through wp_validate_redirect() previously, but might as well double-check.