PRfD - MVP Improvements

[itaigilo]

Listing here improvements that could've been part of the MVP, but ended up out-of-scope.
These can bring nice immediate value to the PRfD MVP users, with relatively scoped effort.

Features

• Prevent from opening a PR on src-dest branches that already have an open PR.
• Unify Compare tab with Branches tab (now that we have the PRs tab, the comparison could be a button inside Branches).
• Split PR Creation into branches selection followed by adding the PR details.
• Re-open a closed PR.
• List all changes of a merged/closed PR (currently we only link to the merged commit).
• Add List filters and search.