Make blob viewer lock button group smoothly with other buttons

What does this MR do and why?

This MR makes the outermost element of the ee/ lock button component into a <gl-button> so that when it is used alongside other buttons in a <gl-button-group>, its edges will adapt properly.

Screenshots or screen recordings

before	with this MR

How to set up and validate locally

to see the issue:

1. git checkout master
2. echo "Feature.enable(:refactor_blob_viewer)" | rails c
3. find a lockable file in a repo
4. observe the curved right side of the lock/unlock button

to validate the solution:

1. git checkout mfluharty-blob-viewer-button-group
2. echo "Feature.enable(:refactor_blob_viewer)" | rails c
3. find a lockable file in a repo
4. observe the straight line divider between the lock/unlock button and the replace button

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.