Synchronize db/schema.rb

Commit #1c4abc1 altered a migration without updating the database
schema; this synchronizes schema.rb with the state after running
migrations (`createdb hound_development && rake db:migrate`).
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
Only run a single repo_sync job at a time

It's currently possible trigger multiple calls to `/repo_syncs`
from the UI, by clicking the refresh link multiple times, or
reloading the page near the start of a sync.

This change ensures that `/repo_syncs` will not enqueue multiple
synchronization jobs at a time.
Fix Security Documentation

Document that we store user's GitHub token
during the "refreshing repos" phase.

Fix documentation to change that
we do not store the token
when enabling repos.

https://trello.com/c/qnFa7l7A/404-fix-security-documentation
Add tasks to remove duplicate data
Upgrade to cedar-14 stack
Remove error handling from GithubApi#create_team

This was introduced by 383c76a to fix an issue when caching against
GitHub, but that is no longer enabled.
Update status on GitHub to show Hound activity

https://trello.com/c/WZ2ERUc7/420-use-github-status-api-to-report-hound-progress