Add ability to persist custom medication codes

[ashleybot]

Fixes #239

This feature allows end-users to search RxNorm API and add more drug codes to the local lookup (there is a foreign key requirement for A4 RxNormIds to the look up).

It additionally implements several scenarios which could be possible, but does not stretch to encompass all potentially polishes for UX. This will be iterated upon as we learn more.

It implements:

• User adding custom medications to an existing form with already selected medications. RxNormIds are not allowed to duplicate.
• User adding custom medication to an unsaved form. It includes if they skip the first question ANYMEDS.
• User adding custom medications in consecutive order. After successfully finding the custom medication it is added to the A4 and persisted in between redirects.
• If the user attempts to add a custom medication that already exists it will search and use the local medication first.
• Saves any changes along the way to adding custom medications.

It does not implement:

• Polish such as system feedback during slow response times (relies on user not rage clicking)

A4 flow with custom medications