Inconsistent use of sigil, handle and label

[vvvyyynet]

The use of the following variables is inconsistent, which should be documented or fixed at some point: sigil/sigla, humanReadableSigil, handle, label.

In fact they could be reduced to:

1. siglaHandle/siglaHandle: "nk" (for "n klein") or "fr5"
2. sigil/sigla: "n" or "Fr. 5 (d)"

Currently, label and humanReadableSigil (as well as sigla in the searchIndex.json files) are used for (1) whereas handle, sigil/sigla and are used for (2).

The inconsistency originates from the .json files (as stated in @vvvyyynet in #40 (comment)_): metadata-nomenclature.json uses sigla and handle correctly, whereas searchIndex.json incorrectly uses sigla for the handles. Currently this is compensated for in the frontend, introducing confusing name choices.

Originally posted by @vvvyyynet in #40 (comment)