Move custom member roles strings out of constants file

What does this MR do and why?

For the Roles and Permissions page:

There are some strings in a constants.js file. Following the recommendations in this handbook page, this MR moves the strings out of constants.js and into the components that use them.

How to set up and validate locally

1. Go to a top-level group. Go to Settings -> Roles and Permissions.
2. Verify that all the text is shown correctly.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #431713 (closed)