Dependency update

This commit updates the spring boot version used by the install
script and adds 3 new starters: ws, hornetq and elasticsearch
Add html unit tests

This commit integrates with spring-test-htmlunit and provides tests
that use the HTML form.
Restore previous HTML form in a different location

This commit restores the previous HTML page that is currently used by
the STS wizard at a different location. This allows a single instance
to serve both the new UI and the (now) STS-specific form.

The LegacyStsController can be used to serve that endpoint and is not
auto configured. app.groovy has been updated to explicitely import that
bean.

The tests infrastructure has been abstracted a bit so that both pages
are actually tested with a set of common tests.

Fixes gh-33
Add the ability to refresh the metadata

This commit updates all consumers of the InitializrMetadata to go
through an InitializrMetadataProvider instead of having a direct
handle to the metadata instance.

A default InitializrMetadataProvider implementation that looks up
for some metadata from projects.spring.io has also been added.

That method is actually cached with a TTL of 10 minutes. This
allows the service to update itself automatically when such
metadata are updated in Sagan.

Fixes gh-26