Bootstrap the complete representation of all available channels into the learn page using plugin data

[rtibbles]

#8548 bootstraps the channel thumbnail into the page to make it available for use by any contentnode within the channel.

To avoid an additional request, we should bootstrap all available channels into learn's plugin_data and use that instead of an API request.

These can then be bootstrapped into this composable: https://github.com/learningequality/kolibri/blob/release-v0.15.x/kolibri/plugins/learn/assets/src/composables/useChannels.js