Implement context feature for data

[dkammer]

in overlaycontrols.component.html the selection of contexts is disabled because it is not implemented application-wide.

<!-- <label>
    <span class="select-label">Context</span>
    <select [(ngModel)]="selectedContext" (ngModelChange)="emitSettingsChange()">
    <option *ngFor="let context of contexts" [value]="context">{{ context }}</option>
    </select>
</label> -->

This feature needs to be completely re-written and re-implemented in dataprovider, dashboard, etc.