Header not sticky when scrolling

**Is your feature request related to a problem? Please describe.**
When scrolling to the bottom of the page, a user has to scroll all the way back up in order to access the header bar.

**Describe the solution you'd like**
The header bar can be found: `https://github.com/Elsa-Health/ohap/blob/main/app/components/header.imba` and should be made to stick to the top of the page when scrolling down.