Perform validation on preview script

[MarcusNoble]

When selecting preview script the form should first be validated as it would when submitting.

For Tester:

Note: Permissions may need to be altered for this to work as expected.

1. Navigate to ​​http://datareducedev.isis.cclrc.ac.uk/autoreduce_webapp/instrument/GEM/ (logging in if needed).
2. Click the "Add new run variables" link
3. Enter some text into one of the numerical entries.
4. Click "Preview reduction script"
5. Check that the numerical box with text is now highlighted red and that the form didn't submit.
6. Correct the invalid input
7. Click "Preview reduction script"
8. Verify that the values in the downloaded script match those in the form.

[Anders Markvardsen]

Validate happen and red highligting is shown

Minor confusion: If ga ga 'Run number start' is entered, it is the "Finished" box that gets highligthed