Change "Become A Researcher" on landing page

[ojaffe]

Change "Become a Researcher" to something different; researcher applications are closed now.

Change back to "Join a course", sub-text "Sign up to any of our courses here!"

[pluv1]

The banner on the blog page could also be removed

[ojaffe]

Changing my original suggestion to: "Join Our Team!" "Sign up to be a projects exec"

Pointing to this link https://forms.gle/DPuiUXSm33NrQjko9

[Edward-Upton]

Resolve in 006ca47 and 8468c60