Sometimes PullHotlinkedImages pulls down a site emoji and creates a new upload record for it. In the cases where these happen the upload is not created via the normal path that custom emoji follows, so we need to check in UploadSecurity whether the origin of the upload is based on a regular site emoji. If it is we never want to mark it as secure (we don't want emoji not accessible from other posts because of secure media).
This only became apparent because the uploads:ensure_correct_acl rake task uses UploadSecurity to check whether an upload should be secure, which would have marked a whole bunch of regular-old-emojis as secure.
* Because custom emoji count as post "uploads" we were
marking them as secure when updating the secure status for post uploads.
* We were also giving them an access control post id, which meant
broken image previews from 403 errors in the admin custom emoji list.
* We now check if an upload is used as a custom emoji and do not
assign the access control post + never mark as secure.
Add TopicUploadSecurityManager to handle post moves. When a post moves around or a topic changes between categories and public/private message status the uploads connected to posts in the topic need to have their secure status updated, depending on the security context the topic now lives in.
Custom emoji, profile background, and card background were being set to secure, which we do not want as they are always in a public context and result in a 403 error from the ACL if linked directly.
### General Changes and Duplication
* We now consider a post `with_secure_media?` if it is in a read-restricted category.
* When uploading we now set an upload's secure status straight away.
* When uploading if `SiteSetting.secure_media` is enabled, we do not check to see if the upload already exists using the `sha1` digest of the upload. The `sha1` column of the upload is filled with a `SecureRandom.hex(20)` value which is the same length as `Upload::SHA1_LENGTH`. The `original_sha1` column is filled with the _real_ sha1 digest of the file.
* Whether an upload `should_be_secure?` is now determined by whether the `access_control_post` is `with_secure_media?` (if there is no access control post then we leave the secure status as is).
* When serializing the upload, we now cook the URL if the upload is secure. This is so it shows up correctly in the composer preview, because we set secure status on upload.
### Viewing Secure Media
* The secure-media-upload URL will take the post that the upload is attached to into account via `Guardian.can_see?` for access permissions
* If there is no `access_control_post` then we just deliver the media. This should be a rare occurrance and shouldn't cause issues as the `access_control_post` is set when `link_post_uploads` is called via `CookedPostProcessor`
### Removed
We no longer do any of these because we do not reuse uploads by sha1 if secure media is enabled.
* We no longer have a way to prevent cross-posting of a secure upload from a private context to a public context.
* We no longer have to set `secure: false` for uploads when uploading for a theme component.
