Add conflict detector

[CeesJol]

If user uses multiple devices they might run into a conflict. Currently the most recent version is used and overwrites the current resume, leading to potentially many lost saves. To solve this, for each resume, add an index that is incremented on each call to UPDATE_RESUME. If the local counter is less than the one on the DB, instead of updating the resume, inform the user there's a conflict and ask them which version they would like to save. Also inform them that refreshing the page will give them the updated resume.