Add ability to specify alternative text for media attachments (#5123)

* Fix #117 - Add ability to specify alternative text for media attachments

- POST /api/v1/media accepts `description` straight away
- PUT /api/v1/media/:id to update `description` (only for unattached ones)
- Serialized as `name` of Document object in ActivityPub
- Uploads form adjusted for better performance and description input

* Add tests

* Change undo button blend mode to difference

app/javascript/mastodon/actions/compose.js

@@ -37,6 +37,10 @@ export const COMPOSE_COMPOSING_CHANGE = 'COMPOSE_COMPOSING_CHANGE';
 
 export const COMPOSE_EMOJI_INSERT = 'COMPOSE_EMOJI_INSERT';
 
+export const COMPOSE_UPLOAD_CHANGE_REQUEST     = 'COMPOSE_UPLOAD_UPDATE_REQUEST';
+export const COMPOSE_UPLOAD_CHANGE_SUCCESS     = 'COMPOSE_UPLOAD_UPDATE_SUCCESS';
+export const COMPOSE_UPLOAD_CHANGE_FAIL        = 'COMPOSE_UPLOAD_UPDATE_FAIL';
+
 export function changeCompose(text) {
   return {
     type: COMPOSE_CHANGE,
@@ -165,6 +169,40 @@ export function uploadCompose(files) {
   };
 };
 
+export function changeUploadCompose(id, description) {
+  return (dispatch, getState) => {
+    dispatch(changeUploadComposeRequest());
+
+    api(getState).put(`/api/v1/media/${id}`, { description }).then(response => {
+      dispatch(changeUploadComposeSuccess(response.data));
+    }).catch(error => {
+      dispatch(changeUploadComposeFail(id, error));
+    });
+  };
+};
+
+export function changeUploadComposeRequest() {
+  return {
+    type: COMPOSE_UPLOAD_CHANGE_REQUEST,
+    skipLoading: true,
+  };
+};
+export function changeUploadComposeSuccess(media) {
+  return {
+    type: COMPOSE_UPLOAD_CHANGE_SUCCESS,
+    media: media,
+    skipLoading: true,
+  };
+};
+
+export function changeUploadComposeFail(error) {
+  return {
+    type: COMPOSE_UPLOAD_CHANGE_FAIL,
+    error: error,
+    skipLoading: true,
+  };
+};
+
 export function uploadComposeRequest() {
   return {
     type: COMPOSE_UPLOAD_REQUEST,