[bugfix] Fix potential dereference of accounts on own instance

[tsmethurst]

This PR updates some of the logic in GetRemoteAccount to ensure that accounts from our own instance are never webfingered. This has happened before and causes an issue where the domain of the account is set to AccountDomain or Host, which means that the account in question is no longer picked up correctly by GetLocalAccountByUsername, which causes all sorts of knock-on issues.

Rather than trying to track down + add checks for every instance where callers might be passing a local account into this function, it's easier to just add the logic into GetRemoteAccount instead.

Callers should still try to avoid passing local accounts into GetRemoteAccount because it's confusing, but at least if they do it now it won't break the account that was passed in.

To support this update, this PR also adds another index to the account cache, for username and domain lookups, which replaces the db.Where that was being used before. So, should be a little faster in some places too!