Download: Move extra channel metadata import to background

Like the unavailable content thumbnails, move the download of the extra
channel metadata to the background instead of blocking the initial
starter pack download on it.

Helps: #592

kolibri_explore_plugin/collectionviews.py

@@ -50,8 +50,7 @@
 
 PROGRESS_STEPS = {
     "importing": 0.1,
-    "downloading": 0.8,
-    "tagging": 0.9,
+    "downloading": 0.9,
     "completed": 1,
 }
 
@@ -247,8 +246,8 @@ class DownloadStage(IntEnum):
     IMPORTING_CHANNELS = auto()
     IMPORTING_CONTENT = auto()
     APPLYING_EXTERNAL_TAGS = auto()
-    IMPORTING_EXTRA_CHANNELS = auto()
     FOREGROUND_COMPLETED = auto()
+    IMPORTING_EXTRA_CHANNELS = auto()
     IMPORTING_ALL_THUMBNAILS = auto()
     COMPLETED = auto()
 
@@ -444,7 +443,7 @@ def get_status(self):
                 progress = (
                     PROGRESS_STEPS["downloading"]
                     + (
-                        PROGRESS_STEPS["tagging"]
+                        PROGRESS_STEPS["completed"]
                         - PROGRESS_STEPS["downloading"]
                     )
                     * current_task_number
@@ -453,17 +452,6 @@ def get_status(self):
             else:
                 progress = PROGRESS_STEPS["downloading"]
 
-        elif self._stage == DownloadStage.IMPORTING_EXTRA_CHANNELS:
-            if total_tasks_number > 0:
-                progress = (
-                    PROGRESS_STEPS["tagging"]
-                    + (PROGRESS_STEPS["completed"] - PROGRESS_STEPS["tagging"])
-                    * current_task_number
-                    / total_tasks_number
-                )
-            else:
-                progress = PROGRESS_STEPS["tagging"]
-
         elif self._stage >= DownloadStage.FOREGROUND_COMPLETED:
             progress = PROGRESS_STEPS["completed"]
 
@@ -520,7 +508,11 @@ def _set_next_stage(self, user):
             elif self._stage == DownloadStage.APPLYING_EXTERNAL_TAGS:
                 tasks = self._content_manifest.get_applyexternaltags_tasks()
             elif self._stage == DownloadStage.IMPORTING_EXTRA_CHANNELS:
-                tasks = self._content_manifest.get_extra_channelimport_tasks()
+                # Download the remaining channel metadata in the background.
+                for (
+                    task
+                ) in self._content_manifest.get_extra_channelimport_tasks():
+                    self._enqueue_background_task(user, task)
             elif self._stage == DownloadStage.IMPORTING_ALL_THUMBNAILS:
                 # Download the remaining content thumbnails in the background.
                 for (