twitter: replace twitter gem with our own API client.

The twitter gem had several problems:

* It's been unmaintained for over a year.
* It pulled in a lot of dependencies, many of which were outdated. In
  particular, it locked the `http` gem to version 3.3, preventing us
  from upgrading to 4.2.
* It raised exceptions on normal error conditions, like for deleted
  tweets or suspended users, which we really don't want.
* We had to wrap it to provide caching.

Changes:

* Fixes #4226 (Exception when creating new artists entries for suspended
  Twitter accounts)
* Drop support for scraping images from summary cards. Summary cards
  are the previews you get when you link to a website in a tweet. These
  preview images aren't always the best image.

Gemfile

@@ -21,7 +21,6 @@ gem 'capistrano-rbenv'
 gem 'streamio-ffmpeg'
 gem 'rubyzip', :require => "zip"
 gem 'stripe'
-gem 'twitter'
 gem 'aws-sdk-sqs', '~> 1'
 gem 'responders'
 gem 'dtext_rb', git: "https://github.com/evazion/dtext_rb.git", require: "dtext"
@@ -45,6 +44,7 @@ gem 'builder'
 gem 'puma'
 gem 'scenic'
 gem 'ipaddress'
+gem 'http'
 
 # needed for looser jpeg header compat
 gem 'ruby-imagespec', :require => "image_spec", :git => "https://github.com/r888888888/ruby-imagespec.git", :branch => "exif-fixes"