EOL: Add files to keep track of unsupported versions/generic EOL message

This commit contains the first pieces necessary to implement a generic
EOL/please-upgrade message whenever a BSA user selects a LibreOffice
version that has reached EOL.

Notes:

 - The content in end-of-life-message.html should possibly live in
   bug.xhtml, but I think it's best to keep it separate in the
   planning stage.

 - We'll still need to add some javascript to run after the version
   has been changed and to show/hide the EOL message.

 - We may want a more complicated system that can provide more
   granular messages based on specific versions/OSes, but this simple
   system should work well enough for now.