Delete mailer queue because we don't use sidekiq_mailer gem and now the mailer queue doesn't exist any more.

Avoid the background_jobs script killing every process with 'sidekiq' in
its argument string (e.g. 'rake gitlab:sidekiq start') by also catching
the number (2) in 'sidekiq 2.14'.

This syntax is compatible with the current Sidekiq version we use
(2.17.0) and it protects us against future breakage by
https://github.com/mperham/sidekiq/commit/040eda558bc64139c846c781d3de0b79603e3035

Updated script/{web,background_job} to use #!/usr/bin/env bash rather than #!/bin/bash to allow for transparency between OS namely Linux and FreeBSD and for source installs of bash where path is not exported by default for /usr/local/{bin,sbin}