Make the check for whether the 'edit files' admin page should be shown consistent

Check was being done differently in three different places, now changed to consistent function call.
Remove `function_exists( 'is_multisite' )` check

As min requirement for the plugin is nowadays well above WP 3.0, the check for `function_exists` on `is_multisite` is no longer needed, removed these elsewhere.
Changed the check for file editing admin screen to use WP native `current_user_can`
Updated changelog
Merge branch 'master' of git://github.com/Yoast/wordpress-seo into Issue-1197-file-edit-directive
Merge branch 'master' of git://github.com/Yoast/wordpress-seo into Issue-1197-file-edit-directive

Conflicts:
	readme.txt
Merge branch 'master' of git://github.com/Yoast/wordpress-seo into Issue-1197-file-edit-directive
Merge branch 'master' of git://github.com/Yoast/wordpress-seo into Issue-1197-file-edit-directive
Merge branch 'master' of git://github.com/Yoast/wordpress-seo into Issue-1197-file-edit-directive

Conflicts:
	inc/wpseo-non-ajax-functions.php
	readme.txt
Merge branch 'master' of git://github.com/Yoast/wordpress-seo into Issue-1197-file-edit-directive

Conflicts:
	readme.txt
Merge pull request #1209 from jrfnl/Issue-1197-file-edit-directive

Issue 1197 - consistent file edit permission check