Fix Slack slash commands using relative URL

- Fix is for both Slack and Mattermost
- avatar_url actually calls avatar_path from within
Avatarable concern. It does allow to pass an
argument to include the full URL with only_path: false
- Update one of the specs to check this behaviour

changelogs/unreleased/jh-chat-slash-commands-user-avatar-relative-url.yml

+---
+title: Fix Slack slash commands using relative URL
+merge_request: 29160
+author:
+type: fixed

lib/gitlab/slash_commands/presenters/base.rb

@@ -91,7 +91,7 @@ module Gitlab
                 title:        "#{issue.title} · #{issue.to_reference}",
                 title_link:   resource_url,
                 author_name:  author.name,
-                author_icon:  author.avatar_url,
+                author_icon:  author.avatar_url(only_path: false),
                 fallback:     fallback_message,
                 pretext:      custom_pretext,
                 text:         text,

spec/lib/gitlab/slash_commands/presenters/issue_show_spec.rb

@@ -3,7 +3,8 @@
 require 'spec_helper'
 
 describe Gitlab::SlashCommands::Presenters::IssueShow do
-  let(:project) { create(:project) }
+  let(:user) { create(:user, :with_avatar) }
+  let(:project) { create(:project, creator: user) }
   let(:issue) { create(:issue, project: project) }
   let(:attachment) { subject[:attachments].first }
 
@@ -15,6 +16,7 @@ describe Gitlab::SlashCommands::Presenters::IssueShow do
     expect(subject[:response_type]).to be(:in_channel)
     expect(subject).to have_key(:attachments)
     expect(attachment[:title]).to start_with(issue.title)
+    expect(attachment[:author_icon]).to eq(user.avatar_url(only_path: false))
   end
 
   context 'with upvotes' do