* feat: add experimental redirect for Learner Home
As a request by Data, this system repeatably groups users into
experiment groups. Based on end of user ID, users that fall beneath a
threshold (LEARNER_HOME_MFE_REDIRECT_PERCENTAGE) will be redirected to
the Learner Home experience, if enabled. Otherwise, learners will see
the legacy dashboard.
* feat: add learner home redirect percentage setting
Adds LEARNER_HOME_MFE_REDIRECT_PERCENTAGE to common settings to avoid a
failed lookup. Can be overridden through config.
* refactor: change test to not leak implementation
