Read only inplace

In-place import currently fails if any of the input files are not writeable by the current user. Previously this was silently failing in the background, but since the work on auto-close, this has prevented further import steps.

cc: @sbesson 
cF: https://trello.com/c/PGvEC54I/343-broken-in-place-import-on-trout
I suppose we don't centrally define these context keys anywhere as constants.