add new mirror button to dashboard

This adds the button to create a new mirror on the dashboard at the same
place where "new repository" and "new organization" already exist.

Dockerfile for RaspberryPi is added.

Admin should be able to see all organizations

various fixes in gogs dump command

This is follow-up for 56c66ee486 allowing admin
to see private repositories, even when not being member of them.

Now, the dump cmd uses setting.CustomPath and setting.LogRootPath
instead of setting.WorkDir which was kind of broken if the gogs
binary was in a different directory than gogs data.
Additionally, the backup of setting.CustomPath directory is only done
if it exists.

Using a tmp dir makes gogs dump more robust to concurrent runs.

It also permits an easier cleaning of the tmp files (gogs-db.sql and
gog-repo.zip) by just removing the tmp dir.

As a side effect, it partially fix bugs on workdir.
Previously, 'gogs dump' created the archives in the current directory,
and tried to include these archives from the directory where the
gogs binary lies.
ex: if gogs binary is in /usr/bin/gogs, and gogs dump is run from /tmp/,
/tmp/gog-repo.zip is created, but gogs dump tried to include
/usr/bin/gogs-repo.zip.

The dump cmd did not check the return value of the z.AddFile or
z.AddDir when building the final archive.
It caused the dump command to succeed even if an error occurred.
The resulted dump archive could be corrupted/empty.
(errors could be various: removal by a concurrent process, disk full,
bugs in the dump cmd itself)

Fixing Dropzone should accept all files when config is "*/*".

Merge branch 'develop' of https://github.com/gogits/gogs into develop