Support defined ordering of grammatical classes within a single definition

[stevecassidy]

Originally reported by: Steve Cassidy (Bitbucket: stevecassidy, GitHub: stevecassidy)

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Ability to reorder grammatical class groups on sign definition display page (not just definitions within groups). Some signs are more verb-like than noun-like and it would be logical to prioritise display of one or the other in some cases. Current order is still default.

This requires another level of defined ordering in the model - currently there is an index value for definitions, this would need a per-gloss way of defining the ordering of definition types - this is currently hard-wired in the code.

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• Bitbucket: https://bitbucket.org/stevecassidy/signbank/issue/94