Refactor access token validation in `Gitlab::Auth`
Refactor access token validation in `Gitlab::Auth`

- Based on @dbalexandre's review
- Extract token validity conditions into two separate methods, for
  personal access tokens and OAuth tokens.

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