Update repository.php

As per issue MDL-46418, it is possible to set a repository plugin to "Enabled but Hidden" and then click on the "Settings" button.  Doing so causes a Section Error because the plugin, once hidden, will not load in the lib/classes/plugininfo.php/repository.php file (function get_enabled_pluigns) where the function checks for visible plugins.  This causes the plugin not to load in the admin tree, throwing an error when the admin tree looks for repositorysettings<pluginname>.  

A more complete fix would be to distinguish between enabled plugins and visible plugins, as that the two are completely different states.  

For Moodle 2.7, this patch creates a conditional on line 346 of the moodle/admin/repository.php file that checks if the plugin is set to  visible.  If so, then it displays the "Settings" link, and hides it if not.  This forces the user to reset the plugin to "Enabled and Visible" before changes can be made to the plugin.

TESTING
   (difficulty: easy, requires admin access)
   1. From the Administration block: Site Administration>Plugins>Repositories
   2. Click Manage Repositories
   3. Note that Plugins set to "Enabled and Visible" will show the "Settings" Link on the far right column.
   4. Set a plugin to "Enabled but Hidden"
   5. Note that the "Settings" link is now missing.
   6. Reset the plugin to "Enabled and Visible" and the link will reappear with the page reload.