Fetching and Parsing Feeds

* Set dashboards#show as root_path for signed_in users
* Add Feedjira to Gemfile
* Add sources and feed_entries tables to database
* Add fetching and parsing methods to source and feed_entry models
* Show user.timeline on dashboards#show
* User can see sources#index
* User can add new sources
```ruby
find_source(feed) || add_source(user, feed)
```

Could look something like

```ruby
user.sources.create_with(etag: feed.etag, url: feed.url, feed_url: feed.feed_url, last_modified: feed.last_modified).find_or_create_by!(name: feed.title)
```

For which I'd probably pull out a feed params method

```ruby
user.sources.
  create_with(feed_params(feed)).
  find_or_create_by!(name: feed.title)
```