Fix public shared folder page when quota includes external storages

When the quota storage wrapper is used (that is, when the share owner
has a quota set) and "quota_include_external_storage" is enabled when
checking the free space "Filesystem::getFileInfo('', 'ext')" is called,
which requires the file system to have been initialized. This is
explicitly done now in "showShare" similar to what is done in
"downloadShare".

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>