Fix broken webmobile repository URL
Define Spring hyperlink and extlink for Spring doc in OMERO conf.py

- Use new Spring URLs (http://spring.io and http://docs.spring.io)
- Update all links to Spring main page/documentation in the developers page
Use webtagging repository as the application example
Merge pull request #494 from sbesson/rebased/develop/spring_redirect

Define Spring hyperlink and extlink for Spring doc in OMERO conf.py (rebased onto develop)
Merge pull request #493 from sbesson/rebased/develop/webmobile_link

Fix broken webmobile repository URL (rebased onto develop)