Update doc/install/installation.md

Line 295: sudo /etc/init.d/gitlab restart
Results in: "Error, unicorn not running!"
This is because unicorn is not yet running for first boot. I suggest changing this to 'start'.

Line 305: sudo apt-get install nginx
Change command to: sudo apt-get -y install nginx
Allowing automated install to simplify and follow pattern of earlier installs.
Update app/models/repository.rb

Issue #2699 - Download button not functioning. Replaces slashes with underscore in downloads filename
Merge pull request #2723 from ZKjellberg/patch-1

Update doc/install/installation.md
Merge pull request #2726 from McJoppy/download-archive-bug

Update app/models/repository.rb
fix deleting non-existing repo
Check if project.repository before check

Fix for #2776
Using empty_repo? instead

Display message if repository is empty
Merge pull request #2803 from donnykurnia/fix-2776

Improve gitlab:check. Fix 2776