fix(views): move bucket update to extra endpoint

BREAKING CHANGE: The bucket id of the task model is now only used internally and will not trigger a change in buckets when updating the task.

This resolves a problem where the task update routine needs to know the view context it is in. Because that's not really what it should be used for, the extra endpoint takes all required parameters and handles the complexity of actually updating the bucket.
This fixes a bug where it was impossible to move a task around between buckets of a saved filter view. In that case, the view of the bucket and the project the task was in would be different, hence the update failed.

pkg/swagger/swagger.yaml

@@ -870,6 +870,17 @@ definitions:
       task_id:
         type: integer
     type: object
+  models.TaskBucket:
+    properties:
+      bucket_id:
+        type: integer
+      project_view_id:
+        type: integer
+      task_done:
+        type: boolean
+      task_id:
+        type: integer
+    type: object
   models.TaskCollection:
     properties:
       filter:
@@ -3855,6 +3866,53 @@ paths:
       summary: Updates a project view
       tags:
       - project
+  /projects/{project}/views/{view}/buckets/{bucket}/tasks:
+    post:
+      consumes:
+      - application/json
+      description: Updates a task in a bucket
+      parameters:
+      - description: Project ID
+        in: path
+        name: project
+        required: true
+        type: integer
+      - description: Project View ID
+        in: path
+        name: view
+        required: true
+        type: integer
+      - description: Bucket ID
+        in: path
+        name: bucket
+        required: true
+        type: integer
+      - description: The id of the task you want to move into the bucket.
+        in: body
+        name: taskBucket
+        required: true
+        schema:
+          $ref: '#/definitions/models.TaskBucket'
+      produces:
+      - application/json
+      responses:
+        "200":
+          description: The updated task bucket.
+          schema:
+            $ref: '#/definitions/models.TaskBucket'
+        "400":
+          description: Invalid task bucket object provided.
+          schema:
+            $ref: '#/definitions/web.HTTPError'
+        "500":
+          description: Internal error
+          schema:
+            $ref: '#/definitions/models.Message'
+      security:
+      - JWTKeyAuth: []
+      summary: Update a task bucket
+      tags:
+      - task
   /projects/{projectID}/duplicate:
     put:
       consumes: