to allow uploads description (alt text) setting.

* Adds CSP headers to the media proxy endpoint

* Sends `content-disposition: attachment; …` for non-image/video/audio
content types

The default list can be overwritten with `:media_proxy,
:safe_content_types` in the configuration.

* Also now appends the filename to the proxy URL (fixes some mobile apps,
it was requested a while ago)

This reverts commit c1d07da4.

The fake support was superseded by 6e030129 which actually implements the faked filters API.

This change removes the fake support and ensures that the actual implementation is used.

(for compatibility with Mastodon)

Closes #245

mastodon api: support descriptions in media api, add PUT endpoint for updating metadata about a media upload