Add ability to inject HTML content into sidebar.html and sidebar-block.html

We should be able to insert HTML below the links in the sidebar and sidebar-block partials. We do a similar thing with jumbotrons with the custom_jumbotron page parameter.

Here is a scenario which could make use of this feature: eclipsefdn/it/websites/sdv.eclipse.org#8 (closed)

We could add buttons below the links such as one for subscribing to calendars.

According to @cguindon this would warrant a need for being implemented at the hugo-solstice-theme level its something that could be used across many sites.