Improve overlapping event validation

[jamesewoo]

When a user enters a new event and encounters the error "Another entry intersects the specified time period", trying to find the overlapping events can be a confusing experience. The simplest solution is to remove the intersection validation or give the user the option to do so, assuming that wouldn't break some other part of the app.

If the validation needs to stay, we could do one or more of:

• Clarify which type of event is overlapping (feeding, sleep, etc.)
• Provide links to the overlapping events
• Give the user the option to submit anyway and have the system modify events so that there are no overlaps

I think that changing how validation works or how the error is presented could really improve the user experience.

[cdubz]

See also #702 proposing the "submit anyway" option as well.

I'm open to changes here and will review PRs. I just haven't had the time recently to look in to this myself.