Performance Improvements

This issue is meant to contain general discussions relating to performance, and then to track performance issues and planned improvements.

• Improving performance of the timelines by fetching all user relations at once (pleroma!2323 (merged))
• BE: control over user relationship extensions in statuses (pleroma!2342 (merged))
• FE: remove the need for the user relation extensions in statuses. These are only used by PleromaFE and slow things down a lot (pleroma-fe#819 (closed))
• Speed up fetching of posts in PleromaFE. Currently, it seems to wait for some unrelated endpoints to return first. Might even be possible to do it right at the start without waiting for the config.json to return. (No issue yet) pleroma-fe#817
• Pre-fill index.html with relevant json for user timelines so the first call to the account and the account statuses can be skipped. pleroma-fe#818