Move attachments to a new app

[rafalp]

This PR moves attachments to misago.attachments app from misago.threads app.

Fixes #1805 and #1812

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
• Update start thread posting state
• Update post reply posting state
• Update edit posting state
• Resize textareas to accomondate new attachment UI
• Use is_deleted flag on attachments for attachment deletion
• Delete user's attachments
• Delete thread's attachments
• Delete category's attachments
• Upload attachments with PostForm (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 (without JavaScript)
• Delete attachments from new post (with JavaScript)
• Delete attachments from edited post (without JavaScript)
• Delete attachments from edited post (with JavaScript)
• Insert attachments in editor's textarea
• Embed attachments in post body
• Gateway attachment files behind permission check
• Option to use NGINX X-Accel-Redirect to secure downloads
• Use lightbox for viewing attachments
• Open attachments on a dedicated page

Posting forms

• 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

[coveralls]

coverage: 97.114% (+0.1%) from 97.013%
when pulling bffe225 on fix-1805-move-attachemts-to-app
into 2d4adc0 on main.