Fixes to evaluated branch and deployment links

[jlstevens]

Takes the good bits of #95 and makes sure the dev website is fixed now the evaluated branch is recovered.

[jlstevens]

In the end, this PR does these things:

• Updated anaconda-project
• Uses user_fields supported by this version of anaconda-project
• Only installs holoviews for website building if it is not already available.
• Use the new nbsite build to create the correct deployment links (original purpose of this PR)

@jbednar Ready to merge if you agree that the deployment links on https://pyviz-dev.github.io/examples/ are now appropriate (all the ones I checked seemed fine to me)

[jlstevens]

After lots of confusion, I ended up leaving the anaconda-project version alone due to the issue described in anaconda/anaconda-project#255. In short, I've added user_fields to all the projects which should be supported in 0.8.4 but it ends up modifying the anaconda-project.yml during prepare (for some unknown reason) which then causes the checkout of the evaluated branch to abort.

[jlstevens]

The root cause has now been identified. Anaconda project 0.8.4 has a bug fixed in anaconda/anaconda-project#256 whenever user_fields was used. This means we will want to update anaconda-project once the next release is available (presumably 0.8.5?).

[jlstevens]

@jbednar The tests have now gone green again. I think this PR is ready to merge noting that the following issues still need to be tackled in subsequent PRs:

• The opensky and gerrymandering projects had test failures when their anaconda-project.yml files had user_fields added. This will need to be investigated.
• The user_fields field won't be respected till anaconda_project 0.8.5 is released and used in dodo.py.
• There is still some issue around param pinning to get rid of the API deprecation warnings.

I think this PR is worth merging if only to get the deployment link fixes into master ASAP.