Avoid hard-coding links and email addresses

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**TL;DR**: Avoid hard-coding links and email addresses

**Narrative**
As a developer, I want links to help text, etc. to be stored separately from the code. This makes it more straightforward to have multiple instances and for other people to maintain their own versions.

**Acceptance Criteria**

- [ ] All external links in lookit-api are defined outside the templates/views
- [ ] All email addresses in lookit-api are defined outside the templates/views

**Implementation Notes**
Probably set as environment variables. I could see putting links in the database as being even easier to adjust (then we just add an initial migration to set values) but would go with whatever the standard approach is here.

Start by searching lookit-api templates and views for external links and email addresses, then review how configuration is currently handled. Compare environment variables with the database approach described in the implementation notes. Done means all such values are defined outside templates and views, with the chosen configuration approach applied consistently.

Written by the indexing model from the issue text.