Compatibility with rails_admin > 3.1 and rails > 6.1

[oloomoses]

• Convert view from .halm to .erb
• Add main.js as JavaScript entry
• Fix dependency issues

NB:

• Paper trial changeset returns a nil value, and therefor the popup widget will be black. To fix this add

config.active_record.use_yaml_unsafe_load = true

in config/application.rb as discussed in here: (https://stackoverflow.com/questions/72970170/upgrading-to-rails-6-1-6-1-causes-psychdisallowedclass-tried-to-load-unspecif)

@rikkipitt