dashboard: Make Schedule table columns hideable/reorderable

[dnadlinger]

This is very handy to make good use of screen real estate, as e.g. file path and Git revision are often not as important to judge the state of the system at a glance – particularly, as the schedule entries can then fit on a single line each.

[dnadlinger]

For instance instead of showing this in a side dock:

This is much more useful to see what's happening at a glance:

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I've been running with a similar change locally for a few years now, but never quite found the time to clean it up.

[sbourdeauducq]

Needs update of release notes.

[dnadlinger]

Done.