Added the ability to add related sources

[ldhtnp]

Added the ability to essentially link related sources together. When logged in, a button to do so appears next to the existing "Suggest a New Source" button.

More information can be found in issue #1196

[susannasiebert]

I left some comments about some design decisions that may need some further thought. There are also some things I noticed when trying out the new form:

• The icon in the form header needs to be changed to the source icon
• The placement of the "Add a related source" button is not ideal. That space is generally reserved for general actions related to this type of entity while action for a specific entity would appear where the pubmed linkout currently is
• The cancel button is positioned on top of the submit button instead of next to it

There is currently no way to view source links as far as I can tell. Depending on wether we decide that directionality is important we should show both the sources that are linked to the current source as well as the sources that have linked the current source to them (basically you want to be able to see the same link from either source).

We should also think about whether we need some ability to delete source linkages