WT-1693 referral invite panel tweaks

[wen-2018]

One-line summary

This PR addresses R3 feedback on referral page.

Significant changes and points to review

See list in Jira ticket.

Issue / Bugzilla link

https://mozilla-hub.atlassian.net/browse/WT-1693

Testing

http://localhost:8000/en-US/invite/?ref_key=TEST000000000001, total successful invites: 1
http://localhost:8000/en-US/invite/?ref_key=TEST000000000002, total successful invites: 3
http://localhost:8000/en-US/invite/?ref_key=TESTA1B2C3000000, total successful invites: 5
http://localhost:8000/en-US/invite/?ref_key=TEST00000000000A, total successful invites: 12

[stephaniehobson]

The gradient line should not go past the badge that has been earned unless the person has referred more than that number of people.

In this example the first badge should be circled but the line after it should not be filled in (the state in the screen shot would be okay if there were 2 referrals)

In this one the second bad should be circled but the line after should not be filled. (the state in the screen shot would be okay if there were 4 referrals)