Optimize /repos page loading

* Use `/repos.json` to help browsers take advantage of ETag caching
* Include subscription relation for repos to fix N+1 query
* Add Rack::Deflater to compress potentially large responses
Don't double-paginate GitHub responses

Oktokit::Client is always initialized with `auto_paginate: true`,
so no additional pagination work is necessary. In fact, paginating
from Hound caused lists with > 100 results to return duplicate
entries and perform redundant work.
Add membership:cleanup_duplicates task