refactor(media): store filenames, use pre-signed s3/azure URLs, UUIDs

Signed-off-by: Erik Michelson <github@erik.michelson.eu>

docs/content/concepts/index.md

@@ -11,6 +11,12 @@ background information and explanations. They are especially useful for contribu
             <span>Notes</span>
         </div>
     </a>
+    <a href='/concepts/media/'>
+        <div class='topic'>
+            <span>📸</span>
+            <span>Media</span>
+        </div>
+    </a>
     <a href='/concepts/user-profiles/'>
         <div class='topic'>
             <span>🙎</span>

docs/content/concepts/media.md

@@ -0,0 +1,23 @@
+# Media
+
+!!! info "Design Document"
+    This is a design document, explaining the design and vision for a HedgeDoc 2
+    feature. It is not a user guide and may or may not be fully implemented.
+
+Media is the term for uploads associated with a note in HedgeDoc.
+Currently, there's only support for images.
+
+Media files can be saved to different storage backends like the local filesystem, S3, Azure Blob
+storage, generic WebDAV shares, or imgur.
+Each storage backend needs to implement an interface with three methods:
+
+- `saveFile(uuid, buffer, fileType)` should store a given file and may return stringified metadata
+  to store in the database for this upload. The metadata does not need to follow a specific format,
+  and will only be used inside the storage backend.
+- `deleteFile(uuid, metadata)` should delete a file with the given UUID. The stored metadata can
+  be used for example to identify the file on the storage platform.
+- `getFileUrl(uuid, metadata)` should return a URL to the file with the given UUID. The stored
+  metadata can be used to identify the file on the storage platform.
+  The returned URL may be temporary.
+
+Uploads are checked for their MIME type and compared to an allow-list and if not matching rejected.