Rewrite media caching (old image provider)

- Doesn't use pyotherside's image provider feature, for more flexibility
  and simplicity
- Suitable for supporting matrix media events and more later
- Avoid a lot of duplicate files that the old cache created due to
  server not returning what we expect, mistakes in Python/QML code, etc
- Changed file structure
  (e.g. thumbnails/32x32/<mxc id> instead of
   thumbnails/<mxc id>.32.32.crop)

- Backend.wait_until_account_exist: start issuing warnings if the
  function runs for more than 10s, which means in most case a bad user
  ID was passed

- New HMxcImage QML component, used in H(User/Room)Avatar

src/qml/Chat/Timeline/EventContent.qml

@@ -52,9 +52,10 @@ HRowLayout {
 
         HUserAvatar {
             id: avatar
+            clientUserId: chatPage.userId
             userId: model.sender_id
             displayName: model.sender_name
-            avatarUrl: model.sender_avatar
+            mxc: model.sender_avatar
             width: parent.width
             height: collapseAvatar ? 1 : 58
         }