Don't delete ways when part of route/boundary relations

[systemed]

Simple patch to prevent ways being unthinkingly deleted from route or boundary relations.

The user can still delete a way which is part of such a relation by manually removing the way from the relation first, then deleting the way.

Fixes #1461.

[tmcw]

LGTM: it does swing the dial from overly lax to pretty strict, but there's a clear way to act on the disabled-message which is good. @jfirebaugh want to review for merge?

[KathleenLD]

👯

[jfirebaugh]

Looks good to me. Based on the feedback in #1461 I suggest we expand this to at least the outer ways of multipolygon relations.

I'd also like to put in a modifier key escape hatch for experienced editors. E.g. hold down alt or shift to bypass the prohibition. If you want to tackle that let me know, otherwise I can merge this and add it myself later.

[systemed]

No problem - I've added a check for multipolygon outers. I'll let you do the modifier key bit!

[jfirebaugh]

I really confused GitHub with some force pushing, but this was merged.

[systemed]

Excellent, thank you!

[KathleenLD]

Nice work, guys!

[brycenesbitt]

Thank you for this, it was much needed, and will save significant future hassle.