Ensure merge request header strings are internationalized

What does this MR do and why?

I recently changed my language to German and realized there were some strings that weren't being translated. I realized it wasn't because translations weren't available, just that the strings themselves weren't internationalized.

All strings existed already with the exception of 'Edit merge requests' (plural). We already have a similar one for 'Edit issues' bulk actions so it makes sense to do the same here.

Screenshots

Before

After (NOTE: 'Mark as ready' is not yet approved as a German translation which is why it's not translated in this screenshot. Similarly for 'Edit merge requests' since it's a new string.)

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.