Fast forward mail_room to include oauth gem upgrade

Fast forwards gitlab-mail_room gem to incorporate upstream changes to oauth gem.

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Getting mailroom running locally I ran into an issue, and that's the the oauth versions were no longer matching up.

Use the following diff in gitlab repo:

diff --git a/Gemfile b/Gemfile
index 7a6e2a13c8c..538ae4c8a0d 100644
--- a/Gemfile
+++ b/Gemfile
@@ -441,7 +441,7 @@ gem 'octokit', '~> 4.15'

 # Updating this gem version here is deprecated. See:
 # https://docs.gitlab.com/ee/development/emails.html#mailroom-gem-updates
-gem 'gitlab-mail_room', '~> 0.0.9', require: 'mail_room'
+gem 'gitlab-mail_room', '~> 0.0.19', require: 'mail_room'

 gem 'email_reply_trimmer', '~> 0.1'
 gem 'html2text'

$ be mail_room -c config/mail_room.yml
Could not find gem 'omniauth-google-oauth2 (~> 1.1)' in locally installed gems.
Run `bundle install` to install missing gems.

Running bundle locally gives the following error:

charlie@hatbox:~/projects/gitlab-development-kit/gitlab$ bundle
Fetching source index from https://rubygems.org/
Resolving dependencies........
Bundler could not find compatible versions for gem "oauth2":
  In Gemfile:
    gitlab-mail_room (~> 0.0.19) was resolved to 0.0.20, which depends on
      oauth2 (~> 1.4.4)

    omniauth-google-oauth2 (~> 1.1) was resolved to 1.1.1, which depends on
      oauth2 (~> 2.0.6)

and I realised it's because oauth gem was upgraded a few months back in gitlab-org/gitlab@933604fd and upstream in https://github.com/tpitale/mail_room/commit/48f8e0822ef407d517671a93a05222bee17af04c.

There is also a config change https://github.com/tpitale/mail_room/commit/14e5bccac93493ad3e84d56140ee172974365dcf that adds delivery_username as an alternate username, but I don't think this will affect our installation since it uses OR.