Implement post editing

[Anonyymi]

• Must be possible via form POST (no javascript)
• Basic futaba style "delete post" form on the bottom right of page, but instead "edit post"
• Extra options: delete file

The UX could be a bit modern in terms of modifying ones post. So the flow could be like this:

• User selects posts to be modified (checkboxes)
• User inputs their password on the form
• User presses "Edit"
• User is brought to a new UI with a list of their posts and options for modifying them
• Modify options to implement: Delete post, Delete file

Later we could add "Modify message", etc. But at this time only "Delete post" is enough.

[Anonyymi]

Not required right now.