Added find group button to sidebar #2379

Summary

Closes #2379 (closed)

3 things

• Add a Find a Group button to the groups sidebar linking to discovery.
• Make Cypress login a data attribute in Cypress
• Fixes a 1px misalignment for icons (the + and Find a Group)in the groups sidebar.

Steps to test

Simple one,

1. Log in
2. Click new button on sidebar
3. Check URL is correct

I've tested this with more than 12 groups.

Estimated Regression Scope

Not a huge change, not much scope for regression but I'd check the sidebar still functions as intended and the discovery URL is correct.