New What we deliver, Contact, and Join pages

This release contains a significant amount of new content, as well as some fine-tuning of content and methods.

New pages:

• [/what-we-deliver/](https://18f.gsa.gov/what-we deliver/) page with new project card layout #2282

Migrated pages:

• join.18f.gov to /join/ page (including an open position for Product Manager) #2326, #2243, #2335
• /hire/ to /contact/.

New content:

• New footer, without newsletter, since the newsletter is in the contact page now #2287

New methods:

• Added the gem jekyll_oembed to streamline the process of embedding link content #2323