Fixes #15861 - increase size of audited_changes column

In mysql text type is limited to 64K, while in psql
there's no limit. Changeing type to MEDIUMTEXT,
which limited to 16MB, should solve this issue in mysql

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 #19612 - CVE-2017-7505 don't expose admin to taxed users

Fixes #19632 - changed shim and grub1 PXE names

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 #18200 - don't re-encrypt settings when value is unchanged

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 #19630 - ignore hidden admins

Fixes #19583 - reworded blank label for subnet