Add "Login to my Solid Pod" in the list of datasources

[rubensworks]

While we already have the option to login with a Solid WebID via the settings, it would be useful to also have an entry in the datasources list.

To quote @pmcb55:

So I was just thinking, could it also be added as an option in the 'Choose datasources:' dropdown menu (just visually, so instead of the current text (when you click on that dropdown) of 'Select a preset datasource, or type custom URL', it becomes 'Select a preset datasource, type custom URL, or log into a Solid Pod', and if you select a 'Solid Pod' option from that dropdown, the UI for 'Solid authentication:' appears.