I can better filter my emails with my MUA

Re-use the syntax already available with story #8940.

Base of the feature

In "Emails notification settings", "Customize email subject" section, there is a text box that allow to define what will be sent. By default the box contains:

• [%{tracker_name} #%{id}] %{title_semantic}

Supported fields type

This needs to be able to handle following fields type:

• ArtifactId
• String

Plus 3 special fields

• %tracker_name for tracker short name
• %_title_semantic bound to semantic title
• %_assigned_to_me (see story #10299) that includes "[Assigned to me]", translated, with the square brackets included when mail is sent to s/o who is assigned to the artifact.

+ Update of the documentation

Pitfalls

• When a field is renamed, the configuration of the notification subject string should be updated
• The value is sent only if the current user being notified has the permission to read the field otherwise an empty string is sent instead of the value.
• If a field has the same name than a special field (eg _title_semantic) then the special field has the precedence