Catch missing commands in Rake tasks

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

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.

Sort projects by activity at group show page

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

Fix Discover Branch

Fixes #1073

envrc files are created when you use direnv (Its recommended in
combination with spring, see: https://github.com/rails/spring#setup)

Use rails helper image-url for reference images in css

Fixes #1001

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

Added part about the help to markdown

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

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

Fixed readme title

Add GFM autocompletion for MergeRequests

Requested
[here](http://feedback.gitlab.com/forums/176466-general/suggestions/5507440-auto-complete-merge-requests-in-gitlab-markdown).