Replace "unresolve discussion" with "reopen discussion"

For our discussions, we use the two phrases Resolve discussion and Unresolve discussion.

Unresolving a discussion is a very technical term that is not something our users would use themselves. A quick Google search showed that the Top 10 results for "Unresolve discussion" are all GitLab pages.

To come closer to the language our users would use themselves, we should switch to a term such as reopen, relaunch or resume.