Allow admin to define the list of providers allowed based on user groups

[callmemagnus]

Black list or White list ?

in any case, it would require:

• an admin page (https://docs.nextcloud.com/server/19/developer_manual/app/settings.html) --- probably not sufficient to implement a form, maybe as a first version with a global setting
• in PageController, set the allowed list in initial state
• only use the intersection between the allowed list and available providers

Issue with white list approach:

a new provider from a newly installed app will not be made available automatically.

To solve:

• [ ] how to retrieve the list of available search providers. It seems to have been only enable from the front-end. If that is the case, an front-end app must be created to push the values from the front-end.

[solracsf]

Having an admin page to disable or enable providers (by groups in a 2nd round) would be a great 1st adition. 👍