Added hide_version_dropdown

[dgarcia360]

Related issue #157

Adds the new template option hide_version_dropdown, which hides a list of tags and branch names from the multi-version dropdown.

For example, if a docs maintainer wants to build docs for the master branch, but does not want to remove master from the sidebar dropdown, they can define in conf.py:

    html_theme_options = {
        'hide_version_dropdown': ['master'],
    }

Once we release this change, I'll go project by project to make sure they are building docs for the default branch and to set up this new option.

How to test this PR

In conf.py, replace BRANCHES = ['master'] with BRANCHES = ['master', 'new-design'] to build docs for multiple versions.

Then, add the new option hide_version_dropdown': ['new-design'] under html_theme_options. For example:

    html_theme_options = {
        'hide_version_dropdown': ['new-design'],
    }

After running make multiversionpreview, you should see in the dropdown only the option "master", but not "new-design".

[dgarcia360]

@lauranovich I've added instructions on the first message to test the PR.

[lauranovich]

something didn't work in the build

I got the following error:
WARNING: unsupported theme option 'hide_version_dropdown' given. Are you sure this goes in the " html_theme_options"

[dgarcia360]

Yes, hide_version_dropdown goes in html_theme_options.

Please run git log and make sure HEAD points to this commit:

If not, please run gh pr checkout 170 to download this PR.