Follow-up: Display link to branch tree explorer after saving changes in the Static Site Editor

We do not display a link to the tree explorer for the branch created to save changes in the Static Site Editor (see !29611 (merged)). We don’t do it because the create branch API do not return the Web URL for the branch.

We should provide the tree explorer base URL to the client and display the branch link again.

Tasks

• Extend create branch API to expose web_url @vyaklushin !30147 (merged)
• (Backup option) Provide a tree_explorer_base_url (feel free to rename according to the any backend best practices) @vyaklushin
• Update the frontend to consume (since we were initially expecting this url, the update will be trivial to wire-up) @derek-knox

@vyaklushin I know we want to not have dual assignees on issues, but this is small enough that we can work in the same branch. In fact when you have the backend portion done, we can pair and do the frontend piece together (unless you want to do that on your own and have me review). For now I will unassign myself until you reassign back or we just pair).