Internal IDs appended instead of summed, causing severe bug

[Mkellyeng]

Context: If the internal ID exceeds 16 digits, floorspace enters a severely bugged state where you can't modify the file any further. 16 digits is a lot though, so it should never be a problem

Problem: When you save, close, and reopen a floorspace JSON, the internal ID counter appends "1" to the highest ID, and continues counting from that new number, adding a digit. In practice this means you can only save, close, reopen a finite number of times.

I've attached 3 floorspace JSON files, in which I've replicated the issue on the web version of floorspace (https://nrel.github.io/floorspace.js/).
Floorspace ID Test 01.json - Created the file, added one space, and saved.
Floorspace ID Test 02.json - Closed and reopened previous, added a second space, and saved.
Floorspace ID Test 03.json - Closed and reopened previous, added a second story, and saved.

Floorspace ID Test 01.json - Highest ID is "13"
Floorspace ID Test 02.json - IDs continue counting from is "131", highest ID is "138"
Floorspace ID Test 03.json - IDs continue counting from is "1381", highest ID is "1384"

Floorspace ID Test.zip

The bugged state:
I don't have an example on hand, but this issue (#377) appears to be in this state, based on what I've seen in the past. You basically can't create anything that uses an ID, so adding a new story or space has the strange double highlighting effect, and you can't draw any new spaces.

[Mkellyeng]

I've tried saving without closing and reopening each time. This did not cause the IDs to append. So it looks like it happens on the 'open'
Floorspace ID Test b.zip

[bgschiller]

Fixed with #379