Flag features with a FIXME tag

[quincylvania]

Features with FIXME or fixme tags, by definition, require attention. As mentioned before in #3130 (comment), iD could flag these issues. KeepRight flags these already, but it would be easy and preferred to add native support.

With #6140, users will be able to view all issues with loaded features. Combined with this warning type, fixmes would be trivial to find.

Newly-added fixmes should not be flagged.

[quincylvania]

I did this! It works well with the Everything validation mode where you can find issues with all downloaded data.

[tordans]

todo might be another key to consider for this.

However, a lot less usage:

• about 1,500,000 for "fixme" https://taginfo.openstreetmap.org/search?q=fixme
• only 39,000 for "todo" https://taginfo.openstreetmap.org/search?q=todo

[quincylvania]

@tordans Interesting. iD should just upgrade todo and TODO to fixme. Possibly FIXME as well.