A VisibilityLevel also can be presented as string
A VisibilityLevel also can be presented as string

For the API, the VisibilityLevel will be exposed as String instead of
Integer. So add the string values and method to translate a level
integer to a string.

t-zuehlsdorff/gitlabhq,darkrasid/gitlabhq,t-zuehlsdorff/gitlabhq,LUMC/gitlabhq,mmkassem/gitlabhq,mmkassem/gitlabhq,dreampet/gitlab,mmkassem/gitlabhq,dplarson/gitlabhq,dreampet/gitlab,jirutka/gitlabhq,LUMC/gitlabhq,stoplightio/gitlabhq,LUMC/gitlabhq,stoplightio/gitlabhq,axilleas/gitlabhq,htve/GitlabForChinese,htve/GitlabForChinese,iiet/iiet-git,darkrasid/gitlabhq,stoplightio/gitlabhq,mmkassem/gitlabhq,iiet/iiet-git,dreampet/gitlab,stoplightio/gitlabhq,axilleas/gitlabhq,dplarson/gitlabhq,dplarson/gitlabhq,jirutka/gitlabhq,iiet/iiet-git,htve/GitlabForChinese,jirutka/gitlabhq,htve/GitlabForChinese,dplarson/gitlabhq,t-zuehlsdorff/gitlabhq,axilleas/gitlabhq,darkrasid/gitlabhq,iiet/iiet-git,LUMC/gitlabhq,axilleas/gitlabhq,darkrasid/gitlabhq,jirutka/gitlabhq,t-zuehlsdorff/gitlabhq,dreampet/gitlab