Store identifiers from other databases #478

The database already contains mountain project IDs. These are useful for correctly associating ascents with routes when importing ticks from mountain project.

It would be nice to have identifiers from other databases too. I have Wikidata and TheCrag in mind, but there are many other possibilities.

Wikidata might be the most powerful. It contains a wide variety of structured data, including images, links to Wikipedia articles, links to other databases, etc.