Make clear that the upgrader script does not update gitlab-shell.
Proper wiki restore. Fixes #845
Merge branch 'upgrader' into 'master'

Make clear that the upgrader script does not update gitlab-shell

There are many cases where people end up with a broken update because they don't update gitlab-shell.

For example: https://twitter.com/hgomez/status/537902038907363330

\cc @sytses

See merge request !246
Merge branch 'restore_wikis' into 'master'

Fix wiki restore

Seems 4a5044e302 only fixed git repos restore. This MR provides the same functionality that was introduced in the aforementioned commit, but for wikis.

See merge request !247