Add relationship vocabulary listing to API

[wshayes]

Add API endpoint to collect relationship vocabulary: label, value, alias? and allow autocomplete for these.

{
label: 'increases',
value: 'increases',
alias: '->'
}, {
label: 'decreases',
value: 'decreases'
}, {
label: 'directlyIncreases',
value: 'directlyIncreases'
}, {
label: 'directlyDecreases',
value: 'directlyDecreases'
}, {
label: 'causesNoChange',
value: 'causesNoChange'
}, {
label: 'positiveCorrelation',
value: 'positiveCorrelation'
}, {
label: 'negativeCorrelation',
value: 'negativeCorrelation'
}, {
label: 'translatedTo',
value: 'translatedTo'
}, {
label: 'transcribedTo',
value: 'transcribedTo'
}, {
label: 'isA',
value: 'isA'
}, {
label: 'subProcessOf',
value: 'subProcessOf'
}, {
label: 'rateLimitingStepOf',
value: 'rateLimitingStepOf'
}, {
label: 'biomarkerFor',
value: 'biomarkerFor'
}, {
label: 'prognosticBiomarkerFor',
value: 'prognosticBiomarkerFor'
}, {
label: 'orthologous',
value: 'orthologous'
}, {
label: 'analogous',
value: 'analogous'
}, {
label: 'association',
value: 'association'
}, {
label: 'hasMembers',
value: 'hasMembers'
}, {
label: 'hasComponents',
value: 'hasComponents'
}, {
label: 'hasMember',
value: 'hasMember'
}, {
label: 'hasComponent',
value: 'hasComponent'
}, {
label: 'actsIn',
value: 'actsIn'
}, {
label: 'includes',
value: 'includes'
}, {
label: 'translocates',
value: 'translocates'
}, {
label: 'hasProduct',
value: 'hasProduct'
}, {
label: 'reactantIn',
value: 'reactantIn'
}, {
label: 'hasModification',
value: 'hasModification'
}, {
label: 'hasVariant',
value: 'hasVariant'
}];

[abargnesi]

This could be implemented as a relationship resource collection via /api/relationships. Completion could be accomplished via filter options provided. Something like the following:

# All relationship resources.
GET /api/relationships

# Single relationship resource.
GET /api/relationships/increases

# Filter relationship resource collection.
GET /api/relationships?filter={"category": "fts","name": "search","value": "hasC"}

It will be important to version by BEL language. My initial thought here would be to use media types for the BEL version.