Recreate full URLs from partial URLs

Summary:
With https://phabricator.kde.org/D8843 applied it might happen that e.g.
the "remoteid" of an Event is not containing the hostname and schem
components of the Events URL. In order for deletes and updates to work
correctly we need reconstruct the full URL before sending any (non
multi-get) requests.

Related Bug: https://bugs.kde.org/show_bug.cgi?id=386985

Reviewers: KDE PIM, knauss

Reviewed By: KDE PIM, knauss

Subscribers: knauss, KDE PIM

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D8844

BUG: 386985
FIXED-IN: 5.7.0