* Remove "Add a private content source" sidebar link

* Update groupsSentence to cover different number of groups

Different cases:
1 group: You have access to the following sources through the group Default.
2 groups: You have access to the following sources through the groups Default and Engineering. (no comma before 'and')
3+ groups: You have access to the following sources through the groups Default, Marketing, and Engineering. (comma before 'and')

* Create PrivateSourcesLayout as a copy of Layout

In this commit PrivateSourcesLayout is a full copy of Layout.
It's going to be updated in later commits.

* Remove unused code from PrivateSourcesLayout

* Update read-only mode warning copy for end-users

* Move copy to constants file

* Add width styles to new sidebar

Also moved private_sources_layout styles to sources.scss file. Having separate files for that little amount of custom styles is unnecessary.

* Move top-level header to sidebar

* Add missing padding to sidebar

* Replace ViewContentHeader with ContentSection props

* Move variables over the components

* Remove unused classnames

* DRY out privateSourcesTable and privateSourcesEmptyState

by extracting privateSourcesSection that contains common markup.

* DRY out sharedSourcesTable and sharedSourcesEmptyState

by extracting sharedSourcesSection that contains common markup.

* Reorder code blocks inside the file

To match the order the components appear in UI

* Add newline to the groups enumeration sentence

So it looks good with any number of groups

* Update x-pack/plugins/enterprise_search/public/applications/workplace_search/views/content_sources/private_sources_layout.tsx

Co-authored-by: Scotty Bollinger <scotty.bollinger@elastic.co>

Co-authored-by: Scotty Bollinger <scotty.bollinger@elastic.co>
Co-authored-by: Kibana Machine <42973632+kibanamachine@users.noreply.github.com>