template: gently web page title warning

* Print a gentler bootstrap-themed warning in legacy pages.

Signed-off-by: Leonard Rossi <leonardo.r@cern.ch>
Please link directly to issues: 

- https://github.com/inveniosoftware/invenio/issues

and perhaps even better to issues for *next*:

- https://github.com/inveniosoftware/invenio/issues?q=is%3Aopen+is%3Aissue+label%3Anext