* OEmbed support for PreviewCard

* Improve ProviderDiscovery code failure treatment

* Do not crawl links if there is a content warning, since those
don't display a link card anyway

* Reset db schema

* Fresh migrate

* Fix rubocop style issues
Fix #1681 - return existing access token when applicable instead of creating new

* Fix test

* Extract http client to helper

* Improve oembed controller

* 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 possibility of unrightful webfinger redirect

* Add more tests for FollowRemoteAccountService

* Refresh local info for remote accounts when webfinger returns new values

It only refreshes account info if one of the URLs or the public-key changes,
in which cases it refreshes the full info, re-downloading the feeds from that
user.

Some special handling should probably be done when the public key changes,
but I have been unable to find any use for it in Mastodon yet.

* Re-fetch remote users we aren't subscribed to.

This might induce performance issues, we might want to only do that for users
we explicitly attempted to subscribe but failed to.

* Refactor changes

* Do not refresh existing remote account details more than once a day

* Avoid re-fetching webfinger info in tests unless otherwise specified

The async action is send before persist, account.id not yet generated

Pull queue receive 'nil' so no profile update.

created via remote look-up when domains differ but point to the same resource

all required resource links (profile page, salmon, atom feed, magic key)

This is a big one, so let me enumerate:

Accounts as well as stream entry pages now contain Link headers that
reference the Atom feed and Webfinger URL for the former and Atom entry
for the latter. So you only need to HEAD those resources to get that
information, no need to download and parse HTML <link>s.

ProcessFeedService will now queue ThreadResolveWorker for each remote
status that it cannot find otherwise. Furthermore, entries are now
processed in reverse order (from bottom to top) in case a newer entry
references a chronologically previous one.

ThreadResolveWorker uses FetchRemoteStatusService to obtain a status
and attach the child status it was queued for to it.

FetchRemoteStatusService looks up the URL, first with a HEAD, tests
if it's an Atom feed, in which case it processes it directly. Next
for Link headers to the Atom feed, in which case that is fetched
and processed. Lastly if it's HTML, it is checked for <link>s to the Atom
feed, and if such is found, that is fetched and processed. The account for
the status is derived from author/name attribute in the XML and the hostname
in the URL (domain). FollowRemoteAccountService and ProcessFeedService
are used.

This means that potentially threads are resolved recursively until a dead-end
is encountered, however it is performed asynchronously over background jobs,
so it should be ok.

respect hub.lease_seconds (fix #46)