further wallet provider oauth fixes and additional styling fixes

[jlbyrne]

Included here:

• leo update
• getting rid of the reference to callback in the oauth2 api endpoint
• Some small styling fixes
• Fixing remove button on the channel card
• Ensuring only bitflyer option appears when location is Japan
• removing the theme slider for now

[github-actions]

The security team is monitoring all repositories for certain keywords. This PR includes the word(s) "oauth" and so security team members have been added as reviewers to take a look.

No need to request a full security review at this stage, the security team will take a look shortly and either clear the label or request more information/changes.

Notifications have already been sent, but if this is blocking your merge feel free to reach out directly to the security team on Slack so that we can expedite this check.

[github-actions]

The following commits were not verified:
09549f6 (unsigned)