Fixes #19588 - Allow setting custom headers in CSV export

Fixes #19585 - Allow csv export of nested attributes

This is required for example to export host facet attributes.

Fixes #19700 - update rubocop rules

The following changes have been made:

- Performance/RedundantMerge:
changes lines such as:
```not_found_message.merge! :message => options```
to:
```not_found_message[:message] = options```

- converts str.match() to str =~ ()...

Fixes #16159 - Rename mail alerts to not be Puppet specific

Right now, under 'my account', users get emails with alerts about the
status of their hosts regarding configuration management (e.g: if there
was an error, get an alert, or get a weekly summary of all hosts...

Fixes #19034 - add tasks for clearing expired notifications

fixes #19528 - rewrite FiltersHelperOverrides to support autoload

Replaces class-level filter search overrides storage with a query
against Foreman::Plugin, allowing the class to be reloaded and tested
safely. Removes another 'registry' class.

Fixes #19534 - improved orchestration task logging

fixes #19464 - rewrite Dashboard::Manager to support autoloading

Replaces class-level widget storage with a static list of built-in
widgets, then appends the registered plugin widgets to get the full
default list of widgets. As a result, the class can safely be reloaded...

fixes #19317, #10927 - move registries to autoload_once path

Partially reverts 70ce8fe to reinstate autoload_once_paths as the
preferred way to load registry-type objects with class variables
containing data set up during app initialisation (e.g. plugin
registrations, menu items.)...

fixes #19192 - adds mark all as read notification action

This implements missing feature from patternfly ( see http://www.patternfly.org/pattern-library/communication/notification-drawer/#/code/angular)