Remove XSS vulnerability

Removed the unsanitized $_SERVER variables to remove the XSS vulnerability.
Updated generic error message
Merge pull request #1684 from rbeede/master

Remove XSS from examplepage.php. Fixes #1683.
No need to substr() URLs. Fixes #1689

URLs are stored in the DB as `text`, so the limit enforced by MySQL will be 65536 -- no need to enforce anything ourselves.
Avoid notices with incomplete plugin headers
More helpful "URL exists" notice. Fixes #1676
Revert "More helpful "URL exists" notice. Fixes #1676"

Revert 8085ddb -- messages should not contain HTML as they are also
used by the API
Add an empty composer.json to push YOURLS on Packagist

This will allow fetching YOURLS via Composer
Adding a line break for URLs that are very long.
Merge pull request #1748 from magno32/staging

Adding a line break for URLs that are very long.
Merge branch 'master', remote-tracking branch 'upstream/master'