Add support for using a proxy for HTTP requests. Only used for external
requests, such as downloading files or talking to source sites such as
Pixiv or Twitter, not for internal requests, such as talking to IQDB or
Reportbooru.
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* It pulled in a lot of dependencies, many of which were outdated.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.
