Refactor 'application.html.erb' layout to merge initially generated version...

Refactor 'application.html.erb' layout to merge initially generated version with version from dul-hydra; DDR-1636.

Earlier commit replaced initially generated version with the dul_hydra version. This seems like a better approach -- merging the relevant portion of the dul_hydra version into the version initially generated by Rails 5. Still may not have it exactly right; e.g., with the flash messages, but we'll know more about that aspect, I think, when ddr-alerts is integrated ( https://duldev.atlassian.net/browse/DDR-1640 ).