Previews to be automatically built after base branch change

**Is your feature request related to a problem? Please describe.**
A repo that I am working on has previews enabled only for the `master` branch.
I created a PR for merging a `feature-improvement` against a `random` base branch.
Later on, I changed the base branch to `master` but the PR never appeared as a preview in the Amplify console.
I'm assuming that's because the PR was not initially based on the `master` branch.

**Describe the solution you'd like**
Here's how I imagine a happy path:
Let's assume we have previews enabled on `master`.
1. Create a PR to some `random` base branch.
1. Change the base branch of that same PR, so that it is now against the`master` branch.
1. That PR now automatically gets built and appears in the Previews section of the Amplify console.

Start with the pull-request base-branch change flow and the Amplify console's Previews section described in the issue. Trace how a PR targeting a configured branch is detected and built, then verify whether changing its base branch can trigger the same behavior. Done means a PR changed from a random base branch to master is automatically built and appears in Previews.

Written by the indexing model from the issue text.