Subscribe and unsubscribe to talk page topics

[mazevedofs]

Phabricator: https://phabricator.wikimedia.org/T311639

Notes

• Add calls to the subscription APIs to get all active subscriptions and subscribe/unsubscribe to a topic
• Update the talk page cells UI accordingly

Test Steps

1. Go to a talk page, tap the button to subscribe/unsubscribe to a topic
2. Leave the page and when returning, check if your subscriptions are correctly updated

[tonisevener]

Nice work! It's working well in my (limited) testing. I have mostly some small tweaks, and one request to optimistically flip the button state instead of waiting for the fetch to go through. Thanks!