Add field for flag(s) flown to Flagpole preset

[1ec5]

osmlab/name-suggestion-index#4745 added flags to the name-suggestion-index and openstreetmap/iD#8305 will incorporate the resulting presets into iD. The Flagpole preset should have a field for identifying the flag of flags that fly from the flagpole. The two most relevant keys are flag:name and flag:wikidata.

At first glance, brand/brand:wikidata would seem like precedent. However, flag:name is always localized into the local language; unlike brand names, a given language would have its own localized name differing from the English name for almost every flag. This makes a flag:name field less amenable to NSI-powered autocomplete. However, the flag:name field would still be worth including in the preset because plenty of flags are not notable enough to have a Wikidata item (for instance, the flag for a company outside that company’s headquarters).

/cc @bhousel