Use same logo for both Sleipnir for Android & iOS

* Merge the Sleipnir for Android and iOS into "Sleipnir Mobile" as
  the Sleipnir for Android browser logo was recently updated and it
  is now the same as the one used for iOS.

* Move the images of the Sleipnir for Android browser logo into the
  archive and rename them to "Sleipnir Mobile" as that logo was also
  used by some of the other Sleipnir mobile browsers.
Update `CHANGELOG.md`