[e.cash] Center H2 image

Summary: Some instances of the H2 component will require a centered image. Adding an optional paramater to allow for that

Test Plan:
npm run dev
scroll down to Roadmap section on the homepage and check the Header looks correct
To see the full effect of the change try changing the "roadmap" title to something longer, and then try removing the center
prop from the H2, you should see the line image go to the left

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D14070