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/Banners/LeftBanner.qml

@@ -8,7 +8,7 @@ Banner {
     // TODO: avatar func auto
     avatar.userId: chatPage.userId
     avatar.displayName: chatPage.userInfo.display_name
-    avatar.avatarUrl: chatPage.userInfo.avatar_url
+    avatar.mxc: chatPage.userInfo.avatar_url
     labelText: qsTr("You are not part of this room anymore.")
 
     buttonModel: [