Wiki -- Provide meaningful commit message

Description

Currently, the auto-generated commit message for wiki changes is copied from the first commit: ${creators_username} created page: ${pagename}. As a result, the history usually is not very useful:

Proposal

Change the default commit message to: ${modifiers_username} modified page: ${pagename}

Also consider to explicitly ask the user to give a meaningful commit message.

Links / references

It looks like the desired behaviour is already implemented in app/models/project_wiki.rb#L110 but then, as the default message is copied from the previous version (where?), the explicitly given message overrides the desired default message in app/models/project_wiki.rb#L170.