i18n: Updated translation template
i18n: Updated German translation
Merge pull request #184 from Turbo87/i18n

Updated translations and template
model: Added tracking_callsign column to User model
tracking: Don't show "other pilots" table if it's empty
forms: Added tracking_callsign field to "Edit User" form
templates: Use tracking_callsign if available, otherwise initials
Merge pull request #187 from Turbo87/user-callsigns

User-configurable callsigns for the tracking pages