Provide a NavigationLinkBlock Plugin and use Help as an usage example

Problem/Motivation

Providing a Single Link Navigation Block type could be useful for situations where a single item needs to be added to navigation and creating a new menu just for this purpose would be overkill and complex for editors.

A good example of this is the Help link in the current navigation. This is a single link and it was hardcoded in the NavigationUserBlock to avoid creating an extra menu.

If we add this new item, we could migrate Help link to be an usage of this block type and significantly simplify logic in NavigationUserBlock.

Related issues where complexity could be reduced:

#3441576: Investigate using the core "User account menu" in favor of custom Navigation Block for same
#3441118: [PP1] Add the user image to the user menu when available

Steps to reproduce

Current process to add a single link to page Foo in navigation:

1. Go to Add Menu Page and create and arbitrary menu called Bar
2. Edit the Bar menu and add menu item to Foo and its title
3. Go to Navigation Layout page and add a new Bar Menu block
4. Configure the Bar Menu Block to use 1st level and depth of 1
5. Link to Foo is added to Navigation

Proposed resolution

Create a new Block plugin for use in the navigation, which allows for configuring a navigation block with a single link, rather than having to place a single link with the overhead of creating a menu, setting a link within it, then setting up a navigation block. This would alleviate the present hardcoding of the Help link as noted above, simplifying NavigationUserBlock, and potentially other user cases, also noted above.

Due to UX concerns with this approach, e.g. the potential proliferation of many single link blocks, this new block type will not be available to be added via UI. But created to make the work on the related issues smoother.

Remaining tasks

Update proposed resolution
Update change record

User interface changes

None. While we added a new Link block here, it is presently hidden from the general Block layout page as well as the Navigation layout page.

API changes

A new block with machine name navigation_link was added.

Data model changes

None.