Add periods to multiple sentences in merge request widget states

According to the UX Guide, we should avoid using periods in solitary sentences but use them in multiple sentences.

However, the examples aren't very clear about whether we need a trailing period besides saying "Lists or dialogs with multiple sentences".

We don't have periods for existing FF-merge with conflicts, pipeline failure, conflicts when you can't merge, need to resolve discussions, sha mismatch, pipeline blocked, etc