Client does not check URL scheme before opening server links

Server links should only open http(s) links. Currently, it is possible to use other schemes (like file:// to open local files), which probably shouldn't be possible

Steps to reproduce:

• Download the server jar
• Set `bug-report-link` to a `file://` url 
• Start the server and join
• Click the 'Report server Bug' button
• The local file described by the url will be opened in the default application