Do not mention “branch” on administer releases page

Problem/Motivation

In issues like #3125239: Suggest 1.0.x as the first branch to create instead of 1.x and a few reported via Slack, people are bumping into the supported “branches” UI not really being branches. It is all just text processing to make a release series.

Steps to reproduce

Click “Administer releases” on a project page.

Proposed resolution

Update the UI to not use the word “branch.”

Remaining tasks

Make the user interface changes.

User interface changes

• Change column heading from “Branch” to “Series”
• Replace x with * in left column content, for example 1.0.x → 1.0.*

API changes

None

Data model changes

None