Add Status of Missions to Foundation Missions page

[shaunop]

Add status of missions to Foundation Missions page. In addition, make a few small copy tweaks.

Status of missions

• Add 3 rows for Open, In progress, and Done and show count based on GitHub data
• If there are 0 open missions, then show full page status as "Closed"
• If there are 1+ open missions, then show full page status as "Open"
• Add "Status of missions" to the right hand column Contents section

Copy edits

• In the right column, change "Explore opportunities" to "Explore missions"
• In How it works, change "Explore open opportunities on GitHub" to "Explore open missions on GitHub"

Designs in figma

Preview:

[sudheerDev]

PR: #1298