Turn the "select a config" button into a drop down list with view templates

[flekschas]

As suggested by @nvictus:

Current the select a config file selection button is confusing as it looks like a drop down button.

Instead it should open a drop down which lets the user select between choosing either a predefined view config if different layouts and an option to load one's own view config

The templates could include

• 2 column
• 3 column
• etc.

[nvictus]

It could also include a few templates populated with common data (e.g. coordinates and gene annotations for hg19/hg38, mm9/mm10). And templates for the "1D genome-browser" view, and with detail views. Then all the extra examples on the homepage can be safely moved to the "examples" gallery. That way, the homepage can be cleaner having only one viewer instance and it should load faster too.