Reasons for creating this PR

Previously, Skosmos only used the User-Agent header Skosmos without any further information. This does not follow relevant recommendations and conventions, and may cause difficulties in analyzing and controlling web traffic.

This PR adds the version number and https://skosmos.org URL to the User-Agent header used in SPARQL, Linked Data, LOC and WDQS requests. A PHPUnit test verifies that it is generated properly.

The "Skosmos" user-agent header was implemented in #1691. Back then we decided not to include the version number:

I decided not to include the Skosmos version number in the user-agent string based on discussion with other Skosmos developers.

Link to relevant issue(s), if any

• follow-up to Set user agent when performing HTTP requests #1691

Description of the changes in this PR

• add Model.getUserAgent() method that generates the User-Agent header value
• use that method to construct user-agent value in GenericSparql, LinkedDataResolver, LOCResolver and WDQSResolver
• add PHPUnit test to verify that the header value is properly generated

Known problems or uncertainties in this PR

I didn't include a test that would verify that the user-agent setting is actually used in HTTP requests. That would probably be possible with Cypress (by intercepting requests) but IMHO it's a lot of work for little benefit.

Checklist

• phpUnit tests pass locally with my changes
• I have added tests that show that the new code works, or tests are not relevant for this PR (e.g. only HTML/CSS changes)
• The PR doesn't reduce accessibility of the front-end code (e.g. tab focus, scaling to different resolutions, use of .sr-only class, color contrast)
• The PR doesn't introduce unintended code changes (e.g. empty lines or useless reindentation)