Remove asserts from production code

[gravesm]

These are okay in tests, but not something that should generally be used in production code. There are a few cases here where it has been done: https://github.com/MITLibraries/solenoid/search?q=assert&unscoped_q=assert

Just using an if statement will usually make the code cleaner. The most compelling reason to not use asserts is to get more useful exceptions. For example, in https://github.com/MITLibraries/solenoid/blob/master/solenoid/elements/views.py#L90 the better solution will be to just remove any asserts or checks and let the app fail with a KeyError if the item doesn't exist at the time you try to access it.