Streamline the content of notification emails

In notification emails, the actual content of the email is often buried under several blocks of chrome — and may even be truncated or completely missing. Ideally, the notification emails would be like *real emails*: a short message of meaningful text, sent from the author of the change that triggered the notification.

This MR includes the following changes to notification emails:

* Remove much of the chrome (e.g. the "GitLab" header)
* Emphasize the content (no more small, grayed-out content)
* Add missing informations to the emails (issue description in "new issue" email, file name in "diff comment" email)
* Add a consistent "View in GitLab" link in the footer
* The assignee is displayed only if someone is assigned
* Fix a rendering bug when viewing emails with [Zimbra](http://www.zimbra.com/)

We use these patches at [Capitaine Train](http://www.capitainetrain.com), and it has been a surprisingly big productivity boost for us.

![Before and after](http://f.cl.ly/items/3n0P2c2v1P0y011c0D3e/Before%20and%20After.png)

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

LDAP code from EE

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Allow underscore as the first char for project names.

[6.6] Fixes a bug with group member administration

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Group owners were not able to remove any users from their group if they were the only owner.

Catch missing commands in Rake tasks

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Remove Dot Org

Add envrc to git-ignore

Respect custom gitlab_shell path in gitlab:gitlab_shell:check task

Natural version sorting

Add Enterprise Linux

Increase Nginx Push Size

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Fix now-invalid invocation of `run_and_match` in gitlab:check

Fix grammar in zero_authorized_projects view

Must have been left over from a previous revision, but the argument
wasn't used anywhere in the method.

This was causing the gitlab:check task to exit prematurely with the
error "System commands must be given as an array of strings."

Prior to this change, if I customized the config value of
`gitlab.gitlab_shell.path`, the check would ignore this value in favor
of `"~<gitlab.gitlab_shell.ssh_user>/gitlab-shell/"` resulting in a
failed check when the configuration was otherwise valid.

Correct typos in web_hooks help.

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