Add email subscribe/unsubscribe flash messages

[alex9smith]

When a user completes the single page email subscribe or unsubscribe
journey they will be redirected to the page they were on with a message
in the account session flash.

Check for these messages in the flash and render the appropriate success
banner. The pages chosen for phase 1 and phase 2 of the rollout are
either publication or detailed_guide content types, so only add the
partial to these templates. In the future when we roll out to more pages
we can add the partial to the rest of the content types.

Also set the vary header to indicate to Fastly that the success
messages are different variations and should be cached and served
separately. The GovukPersonalisation::ControllerConcern handles this
for us but sets the header to vary on the full account session. This
would result in pages being cached per-user which isn't very helpful in
this case.

Instead, override the set_account_vary_header method to vary on the
flash message instead. The message content won't vary per user so this
will mean we only cache three versions of the page - one without a flash
message, one with the subscribed message and one with the unsubscribed
message.

Trello

Subscribe:

Unsubscribe:

[barrucadu]

@alex9smith Can you rebase this so I can deploy the branch for testing?

[alex9smith]

@barrucadu should be good now

[alex9smith]

I've fixed most of the spacing, column width and content changes requested from design review.

There is one outstanding which we are deciding to leave for now as it won't be a simple fix. On desktop only, there is a lot of extra spacing underneath the flash message banners:

This does not appear on mobile. To fix, we'll probably need to modify the component so it can take a custom margin_bottom argument, release the components gem, bump the version here then add in a custom margin to reduce the spacing.

[barrucadu]

Looks good to me (well, other than the extra spacing, but that's a separate issue)