Rename Builds tab to CI tab

Description including problem, use cases, benefits, and/or goals

The word "Builds" is confusing for users. "Builds" is used by other CI providers in different ways and our Builds tab has things in it other than builds. Let's avoid the word and emphasize CI instead.

Proposal

1. Rename Builds tab to CI tab
2. Put current Builds view under a sub-tab and call it Jobs
3. Add another sub-tab for a Stages view (from !3703 (merged))
4. Eventually add a sub-tab for Projects when we support multi-project pipelines (#15655 (closed))
5. Eventually ad a sub-tab for Environments when we support environments (#17009 (closed))

Links / references

• GitLab Pipeline discussion started in #3743 (closed)
• Stages view discussion in !3703 (merged)
• Environment discussion in #17009 (closed)
• Mockup of Stages, but called Pipelines in the mockup
• Mockup of Environments