[WIP] Feature/custom ols3

[sneumann]

Hi, this PR on top of #41 adds the option to search a custom OLS3 endpoint defined in the settings sheet. This serves two use case scenarios:

1. There might be OLS-based ontology services out there we don't know about, maybe catering to a small or even only internal user base. They can point OntoMaton to "their" OLS installation
2. The TIB Service has the notion of community-driven "collections" of ontologies, e.g. https://terminology.tib.eu/ts/ontologies?collection=NFDI4CULTURE, which are (will be) also available with the collection parameter in the API path, e.g. http://terminology.tib.eu/NFDI4CULTURE/api . To avoid that every collection needs its own search menue item, users can point OntoMaton to "their" collection via an entry in the Settings sheet. In theory, the functionality could also be achieved via restrictions against the entire TIB service, but those would have to be curated/maintained client-side, while the collections are community-maintained server-side. This PR also includes the open PRs #35, #37, #40 and #41 , we'll try to do some more testing and update once ready. Yours, Steffen