Show a warning when trying to save a key with the same name

[slipo]

What current issue(s) from Trello/Github does this address?
#396

What problem does this PR solve?
Currently you can accidentally save over preexisting keys by using a key name you've used before.

How did you solve this problem?
Warn the user in a confirmation overlay. They can confirm overwriting the key or cancel and choose a different name.

How did you make sure your solution works?
Save a new key "test". Create another key and try to name it "test"

Are there any special changes in the code that we should be aware of?
no

Is there anything else we should know?
no

• Unit tests written?

Overlay looks like this in case language/design changes are needed:

[coveralls]

Coverage remained the same at 40.781% when pulling 8521d76 on slipo:warning-dup-key-name into 77b14d9 on CityOfZion:dev.