Add new Save page box

[danacotoran]

Add save-this-page component to government-frontend.

The work for this was originally done on the components gem.
However, in the meanwhile it was decided that in the early stages of our experiment, we would only be trialling the component on select pages on the government-frontend app. Adding this as an app-level component at first will ensure that we can iterate on it rapidly in the initial stages (if needed). This code is mostly copied over from the original PR with some fairly significant differences in the tests, as a result of the testing libraries being different.

Nothing renders without a page_path parameter. The component also takes two other optional arguments: page_is_saved and signed_in.

Default (or when the user is signed out)

User is signed in and has not saved this page

User is signed in and has already saved this page

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https://trello.com/c/ZTr0V9gB/770-implement-a-re-usable-save-unsave-component-corresponding-backend-code-in-government-frontend

[owenatgov]

Given this a quick frontend sweep and left some non-blocking comments. Great work 👍