Docker Improvmement and small fix

- Resolve #1893
- Add syslogd to output sshd log on stdout (via `docker logs`)
- Enforce directory rights on `/data/git`, `/data/gogs` & `/data/ssh`
- Add `SOCAT_LINK` environment variable to prevent the creation of scout links when they are not needed (see #1815)

This reverts commit 94b2816446.

move minimum key sizes to config

adding extra check on logpath

Translation for invalid milestone due date format ISO 8601 fix.

Add go-source directive to repo pages

Fix the JSON field names for MigrateRepoForm

This makes Gogs repos work with godoc.org: https://github.com/golang/gddo/wiki/Source-Code-Links

This moves the minimum key sizes into the config file, so that anyone
can modify the restrictions.

For some reason, the field names for Private and Mirror got swapped.