Improve popup message for Git-related actions

When user do some the versioning (Git-related) actions, message are shown in the GUI as popup.

• Ideally they should be in a internationalization file (we are not really good at this, for now).
• There is inconsistency in capitalization. For example: "Your changes are committed and pushed" versus "Commit & Push Done!".
• Why exclamation mark for some, and not the others?

The whole block of message should be updated in regards to our GUI guidelines. You can see the entire message list here: https://github.com/nuxeo/nuxeo-studio/blob/b984e752a46b0d0b7df9447ff49c8008b8ef9711/nuxeo-studio-ui/src/main/java/com/nuxeo/studio/client/event/VersioningActionEvent.java#L21-L34

Also the message in the confirmation dialog in https://github.com/nuxeo/nuxeo-studio/blob/e5bef84103832fe2fc89d1f85c81eed62a153db2/nuxeo-studio-ui/src/main/java/com/nuxeo/studio/client/sourcecontrol/ReleasesAndTagsEditor.java