This issue resolves the following user stories:

1. As a site admin, I can enable MoodleNet sharing for multiple MoodleNet instances, so that teachers can select from a predefined list of MoodleNet instances instead of being restricted to one.
2. As a teacher, I can select which predefined MoodleNet instance to share course content to, so that it is shared to the most appropriate place.

Requirements

1. Update the moodlenet | oauthservice setting in MoodleNet Outbound Settings, so that it supports multi-select where more than one MoodleNet OAuth provider is available in the list ("None" should also still be a valid option).
2. For existing sites, we need to ensure the previously selected single option is retained (if applicable).
3. The Share to MoodleNet modal will need to be updated so that if multiple MoodleNet sites are configured, teachers can select which they wish to share to.

Requirements to be finalised

1. The final UX for admins and teachers.
2. How we determine the default, where multiple are enabled.

Out of scope

1. Teachers sharing to an arbitrary MoodleNet URL (currently only supporting those specified by site admin).