lib-user: Use a getDefaultDateRange() utility

[goplayoutside3]

Package

lib-user

Linked Issue

• This is working toward an ADR and generally more documentation about how we handle date ranges for user stats and project stats.

Describe your changes

• Define getDefaultDateRange() utility function to replace falsey date ranges like { endDate: undefined, startDate: undefined }.
• I started this PR by documenting getDateInterval()'s handling of Javascript Dates and UTC, but realized it's much cleaner to separate out the definition of a default date range from getting the interval for the ERAS query (days, week, month, year).

How to Review

• There's no changes needed for unit tests so all should be passing regardless of timezone
• No UI changes either. I tested this by comparing app-root run locally and viewing my stats, homepage, and groups with no noticeable difference in the UI.

Checklist

PR Creator - Please cater the checklist to fit the review needed for your code changes.
PR Reviewer - Use the checklist during your review. Each point should be checkmarked or discussed before PR approval.

General

• Tests are passing locally and on Github
• Documentation is up to date and changelog has been updated if appropriate
• You can yarn panic && yarn bootstrap or docker-compose up --build and FEM works as expected
• FEM works in all major desktop browsers: Firefox, Chrome, Edge, Safari (Use Browserstack account as needed)

[coveralls]

coverage: 77.767% (+0.04%) from 77.729%
when pulling ad43f72 on default-date-range
into f54edb8 on master.