Adjust the verbiage when reverting a revision

[phenaproxima]

Fixes https://www.drupal.org/node/2854057.

This takes a slightly advanced approach; if the target revision is a forward revision, this will say "switch" instead of "revert". Otherwise, it'll still say "revert". Didn't add tests since this is such a cosmetic change, but could do that if desired.

[balsama]

From 2854057:

If you follow the steps in the original issue, the expected result is:

• Forward revision is set as default revision
• Default revision is unpublished
• Six total revisions remain (image in IS shows six)

Actual results

• Default revision remains unchanged
• New forward revision created that is a clone of the forward revision we wanted to make default