Rename issuable to IssueBase

Given the the Issuable class will soon also have to support MRs this
signals that it can only be used by issues.

lib/gitlab/chat_commands/presenters/issuable.rb → lib/gitlab/chat_commands/presenters/issue_base.rb

 module Gitlab
   module ChatCommands
     module Presenters
-      module Issuable
+      module IssueBase
         def color(issuable)
           issuable.open? ? '#38ae67' : '#d22852'
         end

lib/gitlab/chat_commands/presenters/issue_new.rb

@@ -2,7 +2,7 @@ module Gitlab
   module ChatCommands
     module Presenters
       class IssueNew < Presenters::Base
-        include Presenters::Issuable
+        include Presenters::IssueBase
 
         def present
           in_channel_response(new_issue)

lib/gitlab/chat_commands/presenters/issue_search.rb

@@ -2,7 +2,7 @@ module Gitlab
   module ChatCommands
     module Presenters
       class IssueSearch < Presenters::Base
-        include Presenters::Issuable
+        include Presenters::IssueBase
 
         def present
           text = if @resource.count >= 5

lib/gitlab/chat_commands/presenters/issue_show.rb

@@ -2,7 +2,7 @@ module Gitlab
   module ChatCommands
     module Presenters
       class IssueShow < Presenters::Base
-        include Presenters::Issuable
+        include Presenters::IssueBase
 
         def present
           if @resource.confidential?