[Bugfix] Make filter boosts respect local vs. remote scope

[sneakers-the-rat]

Closes: #37
Continues: #36

The first version of filter duplicate boosts forgot to take the max of only local or remote scopes, so if a post had been boosted more recently by a remote account, the most recent local boost wouldn't be seen and vice versa for the federated feed. This fixes that by including those scopes in the correlated subquery.

Caveats:

• I was able to figure out how to express correlated subqueries using Arel, but try as I might, I wasn't able to figure out how to change the table alias of the contexts from Status when used in the subquery (so rather than WHERE s2.local = true it would always be WHERE statuses.local = true, which didn't work). This means that if the definition of the contexts change, then our subquery will get out of sync, but if that happens the tests should catch us.

Other changes

• Add a multicolumn index on reblog_of_id and id to make subquery faster!
• Move the scope.merge! call out of the block of other scopes so that it's easier to maintain - fewer merge conflicts
• split scope into two scopes to match rest of structure which only has one scope per method
• remove crappy sleep hack to ensure monotonic increase of IDs, just manually increment them.
• explicit rspec contexts for local and remote feeds

I am not sure why the ruby linting is failing now, but i didn't touch any of those files - we'll sort of linter errors when we merge upstream tomorrow