Add translation suggestion based on users known languages

[m-iancu]

The MathHub User module allows users to declare the languages they know. This should be used
to suggest them to translate documents that are missing in multilingual libraries (i.e. smglom).
I.E. user browsing node in language en, his known languages include (en, de) and de version
for this node is missing, then suggest to the user to translate the node to german.
For now that can simply forward to /mh/add-document but we need to improve that later.
See also #84 .

Should probably be added as part of mathhub_user module by implementing hook_node_view and adding some html/js if the conditions described above hold.