supported browsers

To close an old issues I check what is left to e done for #111 "Define supported browsers, add warning for unsupported browsers":

*  [X] Add a clearly noticeable hint on top of the page for older browsers. (Done in #389 and #424)
*  [X] Also add such a hint if Javascript is disabled (Done in 1ce7c78).
*  [X] This policy should be added to the user docs (Done in e8deb35)