Monitor: support accepted discontinuities and discrepancies

[StC-OSM]

The more I use the Monitor function the more I hit freak cases that I would just like to accept as such and ask Knooppuntnet to stop reporting. For instance, there is a discontinuity in EV8 that corresponds to an undeveloped part although the official EuroVelo GPX says otherwise. If you could come up with a solution to handle this it would be great!

[vmarc]

This is related to:

#297 possibility to add comments to deviations (@pelderson)
#289 mechanism to indicate that a deviation is to be ignored (@pelderson)