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 develop -D org -I include -E exclude: PR 450 (omero: Python 2.6 is the minimum supported version)
merge develop -D org -I include -E exclude: PR 493 (Fix broken webmobile repository URL (rebased onto develop))
merge develop -D org -I include -E exclude: PR 494 (Define Spring hyperlink and extlink for Spring doc in OMERO conf.py (rebased onto develop))