Fix: Update timeline header dates

[juliaxbow]

Background

• At some point, we made the decision to denote the first of each month at each timeline tick.
• I'm proposing we revert on this decision following feedback from users that the date ticks are non-intuitive
• I am also suggesting the date ticks extend above the timeline line

Current state:

• Date sits above the timeline tick
• Tick does not extend above the timeline line

Proposed state:

• Date sits between timeline ticks, denoting each section between ticks is a single month
• Tick extends above the timeline line
• Create a timeline header area with fill color and outline; inspect in figma here

[SgtPooki]

This is going to have to wait for #237 because of the way the header is currently rendered and implicitly aligned with the actual milestone cards via the css-grid.