Add link to main app root path in the navbar

[virolea]

Closes #155

Adds a link to the application_selection navbar that redirects to the main_app.root_path:

Note that the main_app.root_path may not be defined, as there are no rules in Rails that enforce an app to have a root path; hence the extra precaution in the code with the check defined?(main_app.root_path).

As a result some users may not understand why the link does not show up in their app, but as suggested in the issue discussion](#155 (comment)), this could be resolved by a configuration option as an iteration of this later on.

I am open on adding it here though, as it shouldn't be too much additional work.

[rosa]

Thanks @virolea, and sorry for the delay!