This avoids injecting the Application god class and assembling default
file locations in multiple places.
In addition, we no longer use the `MountManager` for these uploads. It
only added complexity (by moving tmp files around) and will not be
available in the next major release of Flysystem.
Note: Passing PSR upload streams to Intervention Image requires an
explicit upgrade of the library. (Very likely, users have already
updated to the newer versions, as the old constraint allowed it, but
we should be explicit for correctness' sake.)
Closes#268. Not going to bother with a preview SVG or anything fancy for now – we can think about that as part of #746. Right now it's just good to finally get this functionality in!
Also need to think about apple-touch-icon, msTile stuff, and social sharing image. Not sure if this is all too much for core, but it's definitely too much for the current Appearance page layout. Again, something to think about as part of #746.
Code is a bit rough around the edges, but figured there's not much point in using the command bus properly since #870.
