Attachments v2

[rafalp]

This PR implements new attachments feature as misago.attachments app, making misago.threads smaller. Also brings attachments feature closer to the industry standard.

Fixes #1805, #1813, and #1812

Also does part of the #1801

TODO

• Create misago.attachments app
• Move attachments data from misago.threads
• Make attachment types hardcoded in misago.attachments
• Write tests for the Attachment model
• Move attachments admin to misago.admin
• Validate that image type attachment is supported by Pillow
• Store uploaded file as attachment
• Store image dimensions on model
• Plugin hook for initial attachment plugin_data
• Let admins configure thumbnail dimensions
• Add setting to disable attachments uploads globally
• Add setting to disable attachments uploads in private threads
• Add permission for uploading attachments in private threads
• Pass attachments permissions to the PostForm
• Pass attachments permissions to the PostForm in private threads
• Test PostForm attachments handling logic
• Test posting formsets
• Update start thread posting state
• Update post reply posting state
• Update edit posting state
• Show is_deleted flag in admin
• Management command that deletes flagged attachments
• Management command that deletes old temp attachments
• Utility for deleting attachments
• Utility for deleting user's attachments
• Utility for deleting thread's attachments
• Utility for deleting post's attachments
• Utility for deleting category's attachments
• Test decorator for deleting attachment files at the end of test
• Update delete category actions to use new deletion utils
• Update delete user actions to use new deletion utils
• Include attachments in the GDPR data dump
• Resize textareas to accomodate new attachment UI
• Gateway attachment files behind permission check
• Option to use NGINX X-Accel-Redirect to secure downloads
• Create "category with all relations" fixture with asserts for tests
• Open attachments on a dedicated page
• Limit space for all unused attachments
• Limit space for user's attachments
• Limit space for user's unused attachments
• Delete attachments from their detail page
• Display list of allowed attachment types in admin (as a reference)
• Display attachments storage usage on admin dashboard
• Add setting for naming disallowed file types uploads
• View your attachments space usage in account settings
• Browse your attachments in account settings
• Delete your attachments from account settings
• Display and find broken attachments in admin attachments list
• Add upload API to attachments app
• Add <attachment=filename.jpg:123> syntax to messages parser
• Insert attachments in editor's textarea
• Embed attachments in post body
• Display not embedded attachments under post's body
• Embed attachments in post preview
• Use lightbox for viewing attachments, investigate PhotoSwipe
• Upload attachments with PostForm (without JavaScript)
• Delete attachments from new post (without JavaScript)
• Delete attachments from edited post (without JavaScript)
• Upload attachments with PostForm (with JavaScript)
• Upload attachments with PostForm (JavaScript + paste file)
• Upload attachments with PostForm (JavaScript + drag and drop)
• Delete attachments from new post (with JavaScript)
• Delete attachments from edited post (with JavaScript)
• Replace attachments links in existing posts with new syntax
• Reparse existing posts with new parser (see Reparse existing posts using new parser #1723)

Posting forms

Manually verify and write tests for file uploads and deletion on those forms:

Scenarios to test

• User with permission sees upload UI
• User without permission doesn't see upload UI
• User sees insert attachments UI
• User can upload new attachment without JS
• User can upload new attachment with JS
• User can delete attachment he has permission to

Views to test:

• Start thread
• Start private thread
• Reply thread
• Reply private thread
• Quick reply thread
• Quick reply private thread
• Edit thread
• Edit private thread
• Edit thread post
• Edit private thread post
• Inline edit thread
• Inline edit private thread
• Inline edit thread post
• Inline edit private thread post