Fix grammar and typos in Webhooks help

This MR gets rid of some typos and wrong grammar in [help/web_hooks](https://gitlab.com/help/web_hooks).

Bump gitlab shell

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

Add markdown styleguide.

Add ability to set different ssh host, if different from http/https

Complete admin APIs for SSH Keys

added target_project_id to merge_requests.md

Include SASS in subdirectories with glob.

Break up the long upgrade one liner & add Bash syntax highlighting

Give the Rails cache its own Redis namespace

Missing part of active user

Added description about optional field target_project_id for merge requests.

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