Adjustments for the webservices index page (Bug #1396434)

- Changing the readonly checkboxes to be icon images
- adding a sitewide function display_icon() to render the ticks/crosses
- added a list of generic states to the mahara lang file
- tidied up the varying uses of enabled/disabled to use the generic ones

Change-Id: I009c6d54f0033dd46391c61387c84575536dcd4b
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>