Add source subfield to other fields

[pedromml]

This PR aims to facilitate the tagging of sources for individual fields in the iD editor.
To do it, a new button, labeled "Add source" is was added besides the "Info" button of each field that, when clicked, creates a "subfield" below the main field that allows users to set a value to the tag *:source, like start_date:source
The goal of this is to encourage users to add specific sources to certain fields without them having to manually add the *:source tag.

This is the first PR related to my GSoC project that has the goal of revamping the way users register sources for features.
I'm open to feedback regarding design choices, code style and if the general idea of the update!

This update touches on the *:source tag only, it doesn't change the main "Source" field. An update to that field will be proposed in the future.

ohm_video.mp4

[1ec5]

I’m excited to see this feature taking shape! Thank you for taking the initiative to solicit feedback on the forum. I’ll wait to look more closely at the code until you’re ready for review, but here’s some preliminary design feedback. Take my feedback with a grain of salt: others on the forum might have different opinions; ultimately it’ll be your judgment call.

[1ec5]

This is looking good. I’ve played around with it locally and don’t see any glaring issues. Just a few minor things left, and then we can merge this and move on to the main Sources field and other improvements.

[1ec5]

🎉