allow passing user's email address in custom Gravatar urls

Some custom avatar systems use the user's email address rather than its hash. This merge request will allow the administrator to configure gitlab-ce to use the user's email address gravatar.plain_url and  gravatar.ssl_url in the config file like this:

```
gravatar:
    enabled: true
    plain_url: "http://company.com/avatar/?mail=%{email}&size=%{size}"
    ssl_url: "https://company.com/avatar/?mail=%{email}&size=%{size}"
```

It's a only a 2 word patch.

LDAP Configuration Fix

This fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/180 in new installations of version 6.8 (installations upgraded from 6.7 still have the issue, however).

Fix code block that doesn't render correctly in doc.gitlab.com

Probably fixes https://gitlab.com/gitlab-com/doc-gitlab-com/issues/6

Version680

VERSION to 6.8.0

Update Shell version in the Install doc

Allow to overwrite location of gitlab.yml config file.

Allow to specify secret token via SECRET_KEY_BASE environment variable.

Detailed monthly release

Fixes #1190

Fix minor typo in docs

512mb

Most people only care about the 512mb option, we should mention that with one worked you don't get http pushes.

Update title and revert section of 6.7-to-6.8 docs

fix installation.md typo

depency -> dependency

Workflow documentation

Help pages on doc.gitlab.com/ce are missing titles. This needs to be
solved globally.

This text was deleted in 564c37d9.

Update monthly release doc

/cc @sytse @jacobvosmaer @marin

Official location

I got two today:
https://twitter.com/wonderb0lt/status/456545094644936705
https://twitter.com/madpew/status/456758525398884352

Rc1 updates

#1178

Please check it well 😃

If this gets merged, the annotated version tag 6.8.0.rc1 will point to https://dev.gitlab.org/gitlab/gitlabhq/commit/e6b668ab0169f981a5c4162cd8625daf02e6f5a8

/cc @sytse @jacobvosmaer @marin

fix bundle install error

Removed missing sass selectors

fix issue of bundle install error after slack-notifier upgraded. upgrade to 0.3.2