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Single post view interaction refactorings

Use config.eager_load_paths instead of config.autoload_once_paths

See http://guides.rubyonrails.org/upgrading_ruby_on_rails.html#autoloading-is-disabled-after-booting-in-the-production-environment

Since the gem was renamed, we need to require it manually...

Upgrade to rails 5.1

Otherwise Rails 5 would ignore the migration version and create the
tables with bitints as IDs on MySQL.

we released that in 0.5.0.0 in 2015, we do not support skipping majors
anyway, and this is broken in Rails 5, so let's remove this. If people
upgrade from before 0.5.0.0, they have to upgrade via 0.6.0.0, but
that's written in the documenation.

Although this is contrary to rails best-practises, we cannot provide a schema.rb that works for both MySQL and PostgreSQL, so we have no choice. Our migrations are maintained, so it should always be possible to get back to a "clean" database schema anyway.

... this breaks the Rails 5 upgrade, and it's actually no longer needed.
New installations will have the right size anyway, and even if some
older installations miss the migration by not updating for 2 years, it
still doesn't matter since there is no risk that we will ever have
emojis in our migration filenames.

The change in assets.rb should be enough, but it doesn't work, because
sprockets `after_initialize` runs before it and initializes sprockets
with unfiltered paths.

But the trick with the underscore works, because bootstrap-sass has
named the file `_bootstrap.scss`, and rails-assets-bootstrap has
`bootstrap.scss`, so with `_bootstrap` it uses the correct bootstrap.