Admin Page: ensure user caps are available when picking active tab

[jeherve]

Follow-up from #13541

Changes proposed in this Pull Request:

We need to make sure this.props.userCanManageModules is available before we use it.

I introduced a bug in #13541: when an admin was loading Jetpack > Settings, the Writing tab was loading by default.

Testing instructions:

• Go to Jetpack > Settings as an admin on the site.
• The Security tab and its contents should load by default.
• Go to Users > Add New, and add a new editor to your site.
• Log to your site with that new user.
• Go to Jetpack > Settings.
• The Writing tab and its contents should load instead of the Security tab.

Proposed changelog entry for your changes:

• None

[jetpackbot]

Thank you for the great PR description!

When this PR is ready for review, please apply the [Status] Needs Review label. If you are an a11n, please have someone from your team review the code if possible. The Jetpack team will also review this PR and merge it to be included in the next Jetpack release.

Scheduled Jetpack release: November 5, 2019.
Scheduled code freeze: October 29, 2019

Generated by 🚫 dangerJS against ba67662

[kraftbj]

I think this is tangental. If the secondary user is not connected, it looks kinda broken to me. It may be better to include the CTA to create an account where the settings would go and/or tease what they would get once they connect. I'll open a separate issue for that.