Add client side validation of all validations

Problem/Motivation

The initial validation will be performed on the serverside, however, it will be a much better editorial experience if we can provide the same validation in real time on the client side (javascript).

Acceptance Criteria

• length validation occurs on blur/change
• filename in string validation occurs on blur/change
• rules based validation occurs on blur/change
• prevent and warn style messages are all displayed when the editor leaves the alt text field
• prevent style validation prevents save from happening (probably by disabling the save button)
• warn style failures do not prevent save.

In terms of scope, this will be added after all the server side validation has been added.

Decision Points

1. Decide on the approach:
1. Do we dynamically add the rules and other validations to the javascript so that it is only downloaded once?
2. Do we create a public read only endpoint that lists the rules in json, and the app consumes that, to figure out what is valid
3. Do we make read write endpoint that sends the text to by post and is spits back the failures.
2. This will need UX design