Don't overwrite calendar/task list color with default color

[rfc2822]

When a server doesn't provide a calendar/task list color, we assume a default green.

Currently, this default color is set as calendar color at every sync (when the "Manage calendar colors" setting is active [default]).

Purpose

It's enough to set the calendar color at sync only when the server actually sends a color. So the DAVx5 default color is not set at every sync.

So the when the server doesn't send a color, people can change the calendar color locally in their calendar app (for instance, aCalendar+), and it won't be overwritten at every sync by default.

Short description

A new condition is added to when calendar colors are updated:

• "Manage calendar colors" must be active (like previously)
• The color must be provided by the server (new condition)

Checklist

• The PR has a proper title, description and label.
• I have self-reviewed the PR.
• I have added documentation to complex functions and functions that can be used by other modules.
• I have added reasonable tests or consciously decided to not add tests.

[sunkup]

Looks good 👍