Update presentation for SSO providers on log in page

Overview

Google's given us an ask to update our login page to be in-line with their brand guidelines. In order for us to do this, we need to re-consider how we'd like to present the SSO options on the GitLab login page. Current page:

Current

Sign in	Register

Proposed

Changes introduced:

• changed the styling of social sign in buttons so they match the Google guidelines
• updated Bitbucket logo
• added missing logos
• removed 'Didn't receive a confirmation email?' link
• changed 'Register' button style to primary green
• updated Password input help text so it's up to date with the pattern
• changed the width of CAPTCHA to match the input fields
• we also need to change the 'Name' label to 'Full name' in Profile Settings so it's consistent with what we have here

OP by @williamchia

The Google login logo on our GitLab.com sign-in uses an old logo. Our partner contacts there have asked that we update it.

Brand guidelines:

• Google: https://developers.google.com/identity/branding-guidelines
• Twitter: https://about.twitter.com/content/dam/about-twitter/company/brand-resources/en_us/Twitter_Brand_Guidelines_V2.0.pdf
• GitHub: https://github.com/logos
• Atlassian/BitBucket: https://atlassian.design/guidelines/brand/logos

cc @eliran.mesika @jeremy_