Support filters in the label decoration syntax

In the wake of #566 (closed) (Configurable label decorations for changesets) an of the changes that its implementation led to, it has become obvious that the customization of label decorations by users would be more complete if the keywords supported filters.

In particular, the {message} is currently abbreviated to arbitrary lengths depending on the context:

• 100 characters in changeset labels
• 50 characters in project labels

A more customized syntax maintainers discussed is {message|50}, to imply “message abbreviated to 50 characters”.

A generalization of this syntax would lead to explicitly stating which “filter” is being used. Put differently, the syntax {message|50} would become {message|abbr:50}. This more general syntax allows the subsequent addition of other filters, e.g. uppercase : {message|uppercase} or {author|initials}.

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