[#210] TwitterAPI: alt text support for uploaded images. Mastodon API uploads security fix.

Addresses #210 (closed)

Also addresses !496 (comment 8804) by introducing info[actor] field to uploaded objects, so we can restrict users to setting metadata for owned uploaded objects only (in both Twitter and Mastodon APIs).

Note: per https://developer.twitter.com/en/docs/media/upload-media/api-reference/post-media-upload.html, POST media/upload endpoint does not seem to support passing metadata along with the image data, and https://developer.twitter.com/en/docs/media/upload-media/api-reference/post-media-metadata-create suggests the following request flow:

* Upload media using either the simple upload endpoint or the (preferred) chunked upload endpoint.
* Call this endpoint to attach additional metadata such as image alt text.
* Create Tweet with media_id(s) attached.

This is different from current MastodonApiController#upload implementation which supports uploading image data with description. Per the above flow, this PR implements api/media/metadata/create endpoint which should be used to set uploaded image description.