Customizable greeting in Web UI

[juhoinkinen]

The Annif Web UI has this greetings text:

Welcome!

See the OpenAPI documentation for an interactive REST API specification.

There are some installations of Annif that directly expose the Web UI for public, e.g.: https://data.europa.eu/annif/

For this kind of use the greeting text could be made customizable for giving information about the installation. The html for the greeting is hardcoded in

Annif/annif/templates/home.html

Lines 35 to 37 in b7ecfd2

	<p>Welcome!</p>
	<p>See the <a href="v1/ui/">OpenAPI documentation</a> for an interactive REST API specification.</p>

Vue could be used for this, but maybe there could be some simpler way.