* Make PreviewCard records reuseable between statuses

**Warning!** Migration truncates preview_cards tablec

* Allow a wider thumbnail for link preview, display it in horizontal layout (#4648)

* Delete preview cards files before truncating

* Rename old table instead of truncating it

* Add mastodon:maintenance:remove_deprecated_preview_cards

* Ignore deprecated_preview_cards in schema definition

* Fix null behaviour

* Add spec files for feed and media cleanup workers

* Add coverage for feed and media cleanup schedulers

* Clean up feed and media cleanup workers

* add annotate to Gemfile

* rails g annotate:install

* configure annotate_models

* add schema info to models

* fix rubocop to add frozen_string_literal

In #2110, a new attachment type "unknown" was introduced for
attachments that were rejected due to a domain being blocked using
reject_media. However, the "type" field was never set to "unknown"
because a default value of "0" (image) is set for that column,
causing the `type.blank?` expression to always equal false.

This version uses type_changed? instead, causing the type to be set
to "unknown" unless a type has been explicitly set. This introduces
a small change in behaviour causing the type to be set to unknown
before paperclip calls `before_post_process`. Presumably this
behaviour is more appropriate than the current one because the
attachment type has not been determined by that point.

Included are new tests for `ProcessFeedService` and
`UpdateRemoteProfileService` which now check that remote media is
downloaded for non-blocked domains and is rejected for others.

* Fixes #1985

- add migration AddMediaAttachmentMeta, which add meta field to media_attachments
- before saving attachment, set file meta if needed
- add meta in api

* add spec

* align the “size” format for image and video

* fix code climate

* fixes media_attachment_spec.rb

* Fix #2119 - Whenever about to send a HTTP request, normalize the URI

* Add test for IDN request in FetchLinkCardService

* Perform IDN normalization on domains before they are stored in the DB

* Fix #2108 - Fix gif uploads
Add specs for media attachment gifv conversion

* Add ffmpeg to travis

* Make travis install ffmpeg, not libav

* Switch travis to trusty

* For undownloaded attachments, set type :unknown, display them as a list in the web UI

* Fix case when attachment type is set explicitly

* Fix #1642, fix #1912 - Previous change (#1718) did not modify how original file was saved on upload

* Fix for when file is missing

* Add spec for media controller

* Add MediaAttachment.attached scope

* Simplify methods in media controller

(Because postgres can tell that count(*) needs no extra checks, but
counting a specific column requires them)

before. In the API, attachments now can be either image, video or gifv. Gifv
is to be treated like images in terms of behaviour, but are videos by file
type.

fix ProcessFeedService pushing status into distribution if called a second time
while the first is still running (i.e. when a PuSH comes after a Salmon slap),
fix not running escape on spoiler text before emojify

for local attachments, use them in URLs. Check status privacy
before redirecting to actual file.

for avatars/headers, resize avatars down to 120x120 instead of 300x300. Set cache headers on S3 stuff, also
make it private (aka only accessible via expiring links to prevent hotlinking)

to the API

Using `-strip` cli options from ImageMagick

http://www.imagemagick.org/script/command-line-options.php#strip

fix #85

timeline reload in UI, other small fixes