Validation

[marjev]

1. Fixes small issue that happens because documents and spreadsheets don't have width and height in their embed code

2. Validates whether the url of the Google file is valid (if it returns 200). Validation is on server side and I presumed that user won't edit or type the code in the text area, but will copy-paste it.

[marjev]

Calendar doesn't have to be validated since Google seems to have some internal logic that prevents us from getting Page Not Found result.

Even when no ID is provided, Google returns a calendar.

[martynjames]

1. I think that we need to support a user editing the iframe code - although it is true that the main use case is the copy/paste action.
2. I seem to get a blank screen if no calendar id is provided - I would have imagined google giving us an empty calendar for reference, but that does not appear to be what actually happens
3. Seems to fix word processor documents and spreadsheets problems in previous PR - those fixes should really be applied to the original PR that broke them.
   These 3 PRs could either be:
   i) incorporated into 1 PR, or
   ii) more correctly separated - bugfixes in one, validation in another, event tracking in a third

[marjev]

Should validation be ran only when the Save button is clicked? Right now, validation is ran each time a key is pressed while the focus is on text area, which would in worst case scenario spam the server with validation requests and might confuse the user when the text area gets colored in red before the editing is over.