Add configurable about this dataset links (#907)

* add about arg

* add simple url validator

* attach about link to config api

* add links to configDefaults

* add conditional link in top left and menu item

* whitespace

* change to lower case

* move --about arg before click.command()

if this fixes it I have no idea why

* change link>URL

* be more descriptive about URL

* Make error more explicit

* refactor attach_data to accept about

* format

* change icon

* add trailing parenthesis

* whitespace

server/app/rest_api/rest.py

@@ -65,6 +65,9 @@ class ConfigAPI(Resource):
                     "engine": f"cellxgene Scanpy engine version ",
                     "dataset": current_app.config["DATASET_TITLE"],
                 },
+                "links": {
+                    "about-dataset": current_app.config["ABOUT_DATASET"]
+                },
                 "parameters": {
                     "max-category-items": current_app.data.config["max_category_items"]
                 },