Add Story.Context

[renatomassaro]

Story.Context is used for medium- to long-term persistence of arbitrary Storyline data.

Not to confuse with Story.Manager, which is used for structured data used throughout the entire Storyline. Short-lived data that span only one or a few steps should be persisted on Story.Step meta.

• [x] Automatic (transparent) handling of Helix IDs

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This change is 

[sourcelevel-bot[bot]]

Ebert has finished reviewing this Pull Request and has found:

• 1 possible new issue (including those that may have been commented here).

You can see more details about this review at https://ebertapp.io/github/HackerExperience/Helix/pulls/363.

[renatomassaro]

Reviewed 8 of 11 files at r1, 8 of 10 files at r2, 5 of 5 files at r3, 1 of 1 files at r4. Review status: all files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[renatomassaro]

Reviewed 1 of 1 files at r5. Review status: all files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable