Speed up "show GPG badge" feature specs

Previously, this group of specs would create a new user, add them as a maintainer, sign them in, load the project's commit list, click a specific GPG key badge, and then verify the content in the popover.

Now, we make the project public in order to skip the user setup and login, and load a specific commit page because it loads faster than the commit list while still showing the GPG badge.

8afd0381 - Remove magic SHAs from GPG badge feature spec

We're trying to give the arbitrary SHAs required by each spec a meaningful name.

This also adds an explicit ref definition to each spec so we're not dealing with a mystery guest.

ee172686 - Simplify "changes to verified" GPG feature specs

Previously, we wasted time creating a maintainer and signing them in, then loaded the entire commit list just to verify the GPG status of a single commit.

Now, we rely on the project being public so no user needs to be authenticated, and load just the page for the commit we care about.

87d1e865 - Use have_link over have_content for GPG feature

It's more semantically accurate for what we're verifying.

Baseline

Finished in 54.01 seconds (files took 10.02 seconds to load)
7 examples, 0 failures

After

Finished in 26.31 seconds (files took 8.81 seconds to load)
7 examples, 0 failures

Resulted from https://gitlab.com/gitlab-org/release/framework/issues/67#note_118309347