Client-side Multihoming

**Is your feature request related to a problem? Please describe.**
Not being able to interact with friends due to defederation sucks.

**Describe the solution you'd like**
A way to group multiple accounts (across different instances) under the same client identity would be awesome:

- Clicking a post should fetch it through all accounts, so the replies can be combined (this allows filling in gaps where replies didn't federate to some accounts).
- The home feed should be a merged view of all the accounts' home feeds.
- The local feed should be per-account.
- The community feed should be a merged view of all the accounts' local feeds.
- The federated feed should be a merged view of all the accounts' federated feeds.

Follow requests should still be sent through individual accounts tho. Likewise, replying should require selecting an account to reply from.

**Describe alternatives you've considered**
N/A

**Additional context**
N/A

No files, tests, or entry points are named. Start by locating the account/session and feed handling areas, then map how posts, replies, follows, and account selection currently work; done means implementing the specified merged and per-account views without changing the required account-specific actions.

Written by the indexing model from the issue text.