Fix tags not saving in postgres

Also fixes bug where images can't be viewed in tags.

N.B. The version note field is untouched (even though it shows in the diff - my ide really didn't like this file for some reason and was intent on autoformatting it!!)

## Executive summary
You can't save tags in postgres because of an SQL error. For a related reason in all database formats the images attached to a tag could not be viewed in the frontend

## Detailed description
In the code sprint the images fieldset was moved inside the params ```<field>``` attribute. This meant the name of the images fields changed from ```jform[images][image_intro]``` to ```jform[params][image_intro]```. This meant all images got stored in the params column in the database column leaving the images column blank. As a result no images showed in the frontend on all databases and additionally in postgres the 

## Backwards Compatibility
Any images created on a tag since 3.2 will now not show and will be wiped on next saving the tag (as they are stored in the params column and not the images column as they are supposed to). However offsetting this any images stored between tags introduction in 3.1 and 3.2 WILL now show. 

## Testing Instructions
Test saving a tag in postgres on version 3.3.6 (note NOT current staging because of a regression that was introduced by https://github.com/joomla/joomla-cms/pull/4114)

You can test this on mysql by checking that images now function in the frontend correctly (note in postgres looks like more sql errors mean you can't open a tag yet - that is for a future PR)

## Is Brian Happy yet
You'll have to ask @brianteeman whether this is detailed enough :P