Co-authored-by: Leena Gupte <leena.gupte@digital.cabinet-office.gov.uk>

A new flexible content type called a landing page is being created.
The content type will allow publishers to pick the "blocks" that
should be included on the page, e.g. govspeak, tabs, image, video,
and decide on the order of these "blocks".

As a content schema doesn't exist for the landing_page type yet,
a fake content item has been added to the controller. This will be
removed in a later PR once a content schema exists.

The additional content data path has been created as a constant to
allow it to be overwritten with fixtures in the tests, otherwise we'd
be testing with live content.

This adds fake data for two content items. These will be replaced with
real data in a later PR.

Each of the types of "blocks" will be created in the following commits.

Co-authored-by: Keith Lawrence <Keith.Lawrence@digital.cabinet-office.gov.uk>

It is likely that the different types of blocks will need their own
models for their specific data requirements that builds on the base
block. A "factory" is being used to determine which model to use in a
similar way to that used to build the content item.

However general ancestors are used to check that the block model is of
the correct type rather than a direct ancestor as it will be necessary
to have blocks within blocks to be able to handled different types of
column blocks in later commits.

Co-authored-by: Keith Lawrence <Keith.Lawrence@digital.cabinet-office.gov.uk>

This block doesn't not require any extra data so a new model does not
need to be created for it.

Co-authored-by: Keith Lawrence <Keith.Lawrence@digital.cabinet-office.gov.uk>

A block helper is being used rather than creating a tab model as all
that is changing is how the block data is presented in the tab. Nothing
new is being modelled.

Co-authored-by: Keith Lawrence <Keith.Lawrence@digital.cabinet-office.gov.uk>

This block doesn't not require any extra data so a new model does not
need to be created for it.

Co-authored-by: Keith Lawrence <Keith.Lawrence@digital.cabinet-office.gov.uk>

This will form the foundation for adding blocks containing columns

Co-authored-by: Keith Lawrence <Keith.Lawrence@digital.cabinet-office.gov.uk>

Co-authored-by: Keith Lawrence <Keith.Lawrence@digital.cabinet-office.gov.uk>

This block renders block content in the usual 2/3 and 1/3 proportions.
It uses a theme to control classes on left and right side of blocks.

Co-authored-by: Keith Lawrence <Keith.Lawrence@digital.cabinet-office.gov.uk>

Don't always override with fake data - try getting the content from the
content store first, _then_ fallback to fake if not found.

Allows a blank page with a warning if there isn't a YAML file or blocks
specified in the content item

The routing constraint has been added below account routes otherwise it
causes the account session tests to fail. I think this is due
to how the account content items are stubbed. It seems as though it
hits the first matching route irrespective of document_type.

Ignores the cache-control test for now as this setting comes from the
content item.

Had to convert the response from content store into a hash so that
extra items from YAML files could be merged into it.

Also copies the dummy landing page yaml into a fixture for testing

These tests don't do much as the actual rendering code hasn't
been added yet, so it's difficult to know what should be on the page
to check for.

https://www.youtube.com/watch?v=kpSgaFFPBmk