upgrade to CE v7.5.1 and shell v2.2.0
Merge branch 'jhliberty-master'
install: remove HSTS config from the default headers, Fixes #206
config: rebase nginx configs from gitlab-ce 7.5.1 templates
config: rebase gitlab config from gitlab-ce 7.5.1 templates
config: rebase gitlab-shell configs from gitlab-shell 2.2.0 template
init: added `GITLAB_TIMEZONE` configuration option
init: added `GITLAB_EMAIL_ENABLED` configuration option