Add hook for adding a link to another format to issues/index view

This will be used e.g. for an XLS export plugin.
Moves notification link creation to WP helper
Renames notification link translation
Improve notification link layout
Renames and adapts translation
Renames and adapts issue helper translations
Renames and adapts issue list translation
Renames and adapts issue translation
Renames and adapts priority enum translation
Renames and adapts issue version error translation
Renames and adapts done ration error translation
Renames and adapts WP not found error translation
Moves labels
Renames and adapts my page issue translations
Renames and adapts query and helper translations
Renames and adapts issue labels
Removes superfluous moves controller label
Renames query's issue status labels
Removes superfluous issue relation labels
Renames and adapts no roadmap label